diff options
| -rw-r--r-- | ChangeLog | 9 | ||||
| -rw-r--r-- | manual/filesys.texi | 94 | ||||
| -rw-r--r-- | sysdeps/posix/posix_fallocate.c | 67 | ||||
| -rw-r--r-- | sysdeps/posix/posix_fallocate64.c | 67 |
4 files changed, 199 insertions, 38 deletions
@@ -1,3 +1,12 @@ +2015-06-05 Florian Weimer <fweimer@redhat.com> + + [BZ #15661] + * sysdeps/posix/posix_fallocate64.c (__posix_fallocate64_l64): + Check for overflow properly. Check for O_APPEND. Ignore large + file system block sizes. Add comments about problems. + * sysdeps/posix/posix_fallocate.c (posix_fallocate): Likewise. + * manual/filesys.texi (Storage Allocation): New node. + 2015-06-04 Adhemerval Zanella <adhemerval.zanella@linaro.org> * sysdeps/unix/sysdep.h [SYSCALL_CANCEL]: New macro: define diff --git a/manual/filesys.texi b/manual/filesys.texi index 7d55b43cf2..0f2e3dc3be 100644 --- a/manual/filesys.texi +++ b/manual/filesys.texi @@ -1723,6 +1723,7 @@ modify the attributes of a file. access a file. * File Times:: About the time attributes of a file. * File Size:: Manually changing the size of a file. +* Storage Allocation:: Allocate backing storage for files. @end menu @node Attribute Meanings @@ -3233,6 +3234,99 @@ is a requirement of @code{mmap}. The program has to keep track of the real size, and when it has finished a final @code{ftruncate} call should set the real size of the file. +@node Storage Allocation +@subsection Storage Allocation +@cindex allocating file storage +@cindex file allocation +@cindex storage allocating + +@cindex file fragmentation +@cindex fragmentation of files +@cindex sparse files +@cindex files, sparse +Most file systems support allocating large files in a non-contiguous +fashion: the file is split into @emph{fragments} which are allocated +sequentially, but the fragments themselves can be scattered across the +disk. File systems generally try to avoid such fragmentation because it +decreases performance, but if a file gradually increases in size, there +might be no other option than to fragment it. In addition, many file +systems support @emph{sparse files} with @emph{holes}: regions of null +bytes for which no backing storage has been allocated by the file +system. When the holes are finally overwritten with data, fragmentation +can occur as well. + +Explicit allocation of storage for yet-unwritten parts of the file can +help the system to avoid fragmentation. Additionally, if storage +pre-allocation fails, it is possible to report the out-of-disk error +early, often without filling up the entire disk. However, due to +deduplication, copy-on-write semantics, and file compression, such +pre-allocation may not reliably prevent the out-of-disk-space error from +occurring later. Checking for write errors is still required, and +writes to memory-mapped regions created with @code{mmap} can still +result in @code{SIGBUS}. + +@deftypefun int posix_fallocate (int @var{fd}, off_t @var{offset}, off_t @var{length}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +@c If the file system does not support allocation, +@c @code{posix_fallocate} has a race with file extension (if +@c @var{length} is zero) or with concurrent writes of non-NUL bytes (if +@c @var{length} is positive). + +Allocate backing store for the region of @var{length} bytes starting at +byte @var{offset} in the file for the descriptor @var{fd}. The file +length is increased to @samp{@var{length} + @var{offset}} if necessary. + +@var{fd} must be a regular file opened for writing, or @code{EBADF} is +returned. If there is insufficient disk space to fulfill the allocation +request, @code{ENOSPC} is returned. + +@strong{Note:} If @code{fallocate} is not available (because the file +system does not support it), @code{posix_fallocate} is emulated, which +has the following drawbacks: + +@itemize @bullet +@item +It is very inefficient because all file system blocks in the requested +range need to be examined (even if they have been allocated before) and +potentially rewritten. In contrast, with proper @code{fallocate} +support (see below), the file system can examine the internal file +allocation data structures and eliminate holes directly, maybe even +using unwritten extents (which are pre-allocated but uninitialized on +disk). + +@item +There is a race condition if another thread or process modifies the +underlying file in the to-be-allocated area. Non-null bytes could be +overwritten with null bytes. + +@item +If @var{fd} has been opened with the @code{O_APPEND} flag, the function +will fail with an @code{errno} value of @code{EBADF}. + +@item +If @var{length} is zero, @code{ftruncate} is used to increase the file +size as requested, without allocating file system blocks. There is a +race condition which means that @code{ftruncate} can accidentally +truncate the file if it has been extended concurrently. +@end itemize + +On Linux, if an application does not benefit from emulation or if the +emulation is harmful due to its inherent race conditions, the +application can use the Linux-specific @code{fallocate} function, with a +zero flag argument. For the @code{fallocate} function, @theglibc{} does +not perform allocation emulation if the file system does not support +allocation. Instead, an @code{EOPNOTSUPP} is returned to the caller. + +@end deftypefun + +@deftypefun int posix_fallocate64 (int @var{fd}, off64_t @var{length}, off64_t @var{offset}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} + +This function is a variant of @code{posix_fallocate64} which accepts +64-bit file offsets on all platforms. + +@end deftypefun + @node Making Special Files @section Making Special Files @cindex creating special files diff --git a/sysdeps/posix/posix_fallocate.c b/sysdeps/posix/posix_fallocate.c index d15d60372f..e7fe201b68 100644 --- a/sysdeps/posix/posix_fallocate.c +++ b/sysdeps/posix/posix_fallocate.c @@ -18,26 +18,36 @@ #include <errno.h> #include <fcntl.h> #include <unistd.h> +#include <stdint.h> +#include <sys/fcntl.h> #include <sys/stat.h> #include <sys/statfs.h> -/* Reserve storage for the data of the file associated with FD. */ +/* Reserve storage for the data of the file associated with FD. This + emulation is far from perfect, but the kernel cannot do not much + better for network file systems, either. */ int posix_fallocate (int fd, __off_t offset, __off_t len) { struct stat64 st; - struct statfs f; - /* `off_t' is a signed type. Therefore we can determine whether - OFFSET + LEN is too large if it is a negative value. */ if (offset < 0 || len < 0) return EINVAL; - if (offset + len < 0) + + /* Perform overflow check. The outer cast relies on a GCC + extension. */ + if ((__off_t) ((uint64_t) offset) + ((uint64_t) len) < 0) return EFBIG; - /* First thing we have to make sure is that this is really a regular - file. */ + /* pwrite below will not do the right thing in O_APPEND mode. */ + { + int flags = __fcntl (fd, F_GETFL, 0); + if (flags < 0 || (flags & O_APPEND) != 0) + return EBADF; + } + + /* We have to make sure that this is really a regular file. */ if (__fxstat64 (_STAT_VER, fd, &st) != 0) return EBADF; if (S_ISFIFO (st.st_mode)) @@ -47,6 +57,8 @@ posix_fallocate (int fd, __off_t offset, __off_t len) if (len == 0) { + /* This is racy, but there is no good way to satisfy a + zero-length allocation request. */ if (st.st_size < offset) { int ret = __ftruncate (fd, offset); @@ -58,19 +70,36 @@ posix_fallocate (int fd, __off_t offset, __off_t len) return 0; } - /* We have to know the block size of the filesystem to get at least some - sort of performance. */ - if (__fstatfs (fd, &f) != 0) - return errno; - - /* Try to play safe. */ - if (f.f_bsize == 0) - f.f_bsize = 512; - - /* Write something to every block. */ - for (offset += (len - 1) % f.f_bsize; len > 0; offset += f.f_bsize) + /* Minimize data transfer for network file systems, by issuing + single-byte write requests spaced by the file system block size. + (Most local file systems have fallocate support, so this fallback + code is not used there.) */ + + unsigned increment; + { + struct statfs64 f; + + if (__fstatfs64 (fd, &f) != 0) + return errno; + if (f.f_bsize == 0) + increment = 512; + else if (f.f_bsize < 4096) + increment = f.f_bsize; + else + /* NFS does not propagate the block size of the underlying + storage and may report a much larger value which would still + leave holes after the loop below, so we cap the increment at + 4096. */ + increment = 4096; + } + + /* Write a null byte to every block. This is racy; we currently + lack a better option. Compare-and-swap against a file mapping + might additional local races, but requires interposition of a + signal handler to catch SIGBUS. */ + for (offset += (len - 1) % increment; len > 0; offset += increment) { - len -= f.f_bsize; + len -= increment; if (offset < st.st_size) { diff --git a/sysdeps/posix/posix_fallocate64.c b/sysdeps/posix/posix_fallocate64.c index b845df7a80..ee32679a05 100644 --- a/sysdeps/posix/posix_fallocate64.c +++ b/sysdeps/posix/posix_fallocate64.c @@ -18,26 +18,36 @@ #include <errno.h> #include <fcntl.h> #include <unistd.h> +#include <stdint.h> +#include <sys/fcntl.h> #include <sys/stat.h> #include <sys/statfs.h> -/* Reserve storage for the data of the file associated with FD. */ +/* Reserve storage for the data of the file associated with FD. This + emulation is far from perfect, but the kernel cannot do not much + better for network file systems, either. */ int __posix_fallocate64_l64 (int fd, __off64_t offset, __off64_t len) { struct stat64 st; - struct statfs64 f; - /* `off64_t' is a signed type. Therefore we can determine whether - OFFSET + LEN is too large if it is a negative value. */ if (offset < 0 || len < 0) return EINVAL; - if (offset + len < 0) + + /* Perform overflow check. The outer cast relies on a GCC + extension. */ + if ((__off64_t) ((uint64_t) offset) + ((uint64_t) len) < 0) return EFBIG; - /* First thing we have to make sure is that this is really a regular - file. */ + /* pwrite64 below will not do the right thing in O_APPEND mode. */ + { + int flags = __fcntl (fd, F_GETFL, 0); + if (flags < 0 || (flags & O_APPEND) != 0) + return EBADF; + } + + /* We have to make sure that this is really a regular file. */ if (__fxstat64 (_STAT_VER, fd, &st) != 0) return EBADF; if (S_ISFIFO (st.st_mode)) @@ -47,6 +57,8 @@ __posix_fallocate64_l64 (int fd, __off64_t offset, __off64_t len) if (len == 0) { + /* This is racy, but there is no good way to satisfy a + zero-length allocation request. */ if (st.st_size < offset) { int ret = __ftruncate64 (fd, offset); @@ -58,19 +70,36 @@ __posix_fallocate64_l64 (int fd, __off64_t offset, __off64_t len) return 0; } - /* We have to know the block size of the filesystem to get at least some - sort of performance. */ - if (__fstatfs64 (fd, &f) != 0) - return errno; - - /* Try to play safe. */ - if (f.f_bsize == 0) - f.f_bsize = 512; - - /* Write something to every block. */ - for (offset += (len - 1) % f.f_bsize; len > 0; offset += f.f_bsize) + /* Minimize data transfer for network file systems, by issuing + single-byte write requests spaced by the file system block size. + (Most local file systems have fallocate support, so this fallback + code is not used there.) */ + + unsigned increment; + { + struct statfs64 f; + + if (__fstatfs64 (fd, &f) != 0) + return errno; + if (f.f_bsize == 0) + increment = 512; + else if (f.f_bsize < 4096) + increment = f.f_bsize; + else + /* NFS clients do not propagate the block size of the underlying + storage and may report a much larger value which would still + leave holes after the loop below, so we cap the increment at + 4096. */ + increment = 4096; + } + + /* Write a null byte to every block. This is racy; we currently + lack a better option. Compare-and-swap against a file mapping + might address local races, but requires interposition of a signal + handler to catch SIGBUS. */ + for (offset += (len - 1) % increment; len > 0; offset += increment) { - len -= f.f_bsize; + len -= increment; if (offset < st.st_size) { |