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Fix assorted portability issues with using msync() for data flushing.

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Commit 428b1d6b29 introduced the use of
msync() for flushing dirty data from the kernel's file buffers.  Several
portability issues were overlooked, though:

* Not all implementations of mmap() think that nbytes == 0 means "map
the whole file".  To fix, use lseek() to find out the true length.
Fix callers of pg_flush_data to be aware that nbytes == 0 may result
in trashing the file's seek position.

* Not all implementations of mmap() will accept partial-page mmap
requests.  To fix, round down the length request to whatever sysconf()
says the page size is.  (I think this is OK from a portability standpoint,
because sysconf() is required by SUS v2, and we aren't trying to compile
this part on Windows anyway.  Buildfarm should let us know if not.)

* On 32-bit machines, the file size might exceed the available free
address space, or even exceed what will fit in size_t.  Check for
the latter explicitly to avoid passing a false request size to mmap().
If mmap fails, silently fall through to the next implementation method,
rather than bleating to the postmaster log and giving up.

* mmap'ing directories fails on some platforms, and even if it works,
msync'ing the directory is quite unlikely to help, as for that matter are
the other flush implementations.  In pre_sync_fname(), just skip flush
attempts on directories.

In passing, copy-edit the comments a bit.

Stas Kelvich and myself
pull/12/head
Tom Lane 10 years ago
parent 85e0047077
commit fa11a09fed
1 changed files with 90 additions and 41 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
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  1. 131
      src/backend/storage/file/fd.c

131
src/backend/storage/file/fd.c
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@ -64,6 +64,7 @@
#ifndef WIN32
#include <sys/mman.h>
#endif
#include <limits.h>
#include <unistd.h>
#include <fcntl.h>
#ifdef HAVE_SYS_RESOURCE_H
@ -391,34 +392,36 @@ pg_fdatasync(int fd)
/*
* pg_flush_data --- advise OS that the described dirty data should be flushed
*
* An offset of 0 with an nbytes 0 means that the entire file should be
* flushed.
* offset of 0 with nbytes 0 means that the entire file should be flushed;
* in this case, this function may have side-effects on the file's
* seek position!
*/
void
pg_flush_data(int fd, off_t offset, off_t nbytes)
{
/*
* Right now file flushing is primarily used to avoid making later
* fsync()/fdatasync() calls have a less impact. Thus don't trigger
* flushes if fsyncs are disabled - that's a decision we might want to
* make configurable at some point.
* fsync()/fdatasync() calls have less impact. Thus don't trigger flushes
* if fsyncs are disabled - that's a decision we might want to make
* configurable at some point.
*/
if (!enableFsync)
return;
/*
* XXX: compile all alternatives, to find portability problems more easily
* We compile all alternatives that are supported on the current platform,
* to find portability problems more easily.
*/
#if defined(HAVE_SYNC_FILE_RANGE)
{
int rc = 0;
int rc;
/*
* sync_file_range(SYNC_FILE_RANGE_WRITE), currently linux specific,
* tells the OS that writeback for the passed in blocks should be
* tells the OS that writeback for the specified blocks should be
* started, but that we don't want to wait for completion. Note that
* this call might block if too much dirty data exists in the range.
* This is the preferrable method on OSs supporting it, as it works
* This is the preferable method on OSs supporting it, as it works
* reliably when available (contrast to msync()) and doesn't flush out
* clean data (like FADV_DONTNEED).
*/
@ -438,72 +441,107 @@ pg_flush_data(int fd, off_t offset, off_t nbytes)
#endif
#if !defined(WIN32) && defined(MS_ASYNC)
{
int rc = 0;
void *p;
static int pagesize = 0;
/*
* On several OSs msync(MS_ASYNC) on a mmap'ed file triggers
* writeback. On linux it only does so with MS_SYNC is specified, but
* writeback. On linux it only does so if MS_SYNC is specified, but
* then it does the writeback synchronously. Luckily all common linux
* systems have sync_file_range(). This is preferrable over
* systems have sync_file_range(). This is preferable over
* FADV_DONTNEED because it doesn't flush out clean data.
*
* We map the file (mmap()), tell the kernel to sync back the contents
* (msync()), and then remove the mapping again (munmap()).
*/
p = mmap(NULL, nbytes,
PROT_READ | PROT_WRITE, MAP_SHARED,
fd, offset);
if (p == MAP_FAILED)
{
ereport(WARNING,
(errcode_for_file_access(),
errmsg("could not mmap while flushing dirty data: %m")));
return;
}
rc = msync(p, nbytes, MS_ASYNC);
if (rc != 0)
/* mmap() needs actual length if we want to map whole file */
if (offset == 0 && nbytes == 0)
{
ereport(WARNING,
(errcode_for_file_access(),
errmsg("could not flush dirty data: %m")));
/* NB: need to fall through to munmap()! */
nbytes = lseek(fd, 0, SEEK_END);
if (nbytes < 0)
{
ereport(WARNING,
(errcode_for_file_access(),
errmsg("could not determine dirty data size: %m")));
return;
}
}
rc = munmap(p, nbytes);
if (rc != 0)
/*
* Some platforms reject partial-page mmap() attempts. To deal with
* that, just truncate the request to a page boundary. If any extra
* bytes don't get flushed, well, it's only a hint anyway.
*/
/* fetch pagesize only once */
if (pagesize == 0)
pagesize = sysconf(_SC_PAGESIZE);
/* align length to pagesize, dropping any fractional page */
if (pagesize > 0)
nbytes = (nbytes / pagesize) * pagesize;
/* fractional-page request is a no-op */
if (nbytes <= 0)
return;
/*
* mmap could well fail, particularly on 32-bit platforms where there
* may simply not be enough address space. If so, silently fall
* through to the next implementation.
*/
if (nbytes <= (off_t) SSIZE_MAX)
p = mmap(NULL, nbytes, PROT_READ, MAP_SHARED, fd, offset);
else
p = MAP_FAILED;
if (p != MAP_FAILED)
{
/* FATAL error because mapping would remain */
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not munmap while flushing blocks: %m")));
}
int rc;
return;
rc = msync(p, (size_t) nbytes, MS_ASYNC);
if (rc != 0)
{
ereport(WARNING,
(errcode_for_file_access(),
errmsg("could not flush dirty data: %m")));
/* NB: need to fall through to munmap()! */
}
rc = munmap(p, (size_t) nbytes);
if (rc != 0)
{
/* FATAL error because mapping would remain */
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not munmap() while flushing data: %m")));
}
return;
}
}
#endif
#if defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
{
int rc = 0;
int rc;
/*
* Signal the kernel that the passed in range should not be cached
* anymore. This has the, desired, side effect of writing out dirty
* data, and the, undesired, side effect of likely discarding useful
* clean cached blocks. For the latter reason this is the least
* preferrable method.
* preferable method.
*/
rc = posix_fadvise(fd, offset, nbytes, POSIX_FADV_DONTNEED);
/* don't error out, this is just a performance optimization */
if (rc != 0)
{
/* don't error out, this is just a performance optimization */
ereport(WARNING,
(errcode_for_file_access(),
errmsg("could not flush dirty data: %m")));
return;
}
return;
@ -1510,6 +1548,13 @@ FileWriteback(File file, off_t offset, int amount)
file, VfdCache[file].fileName,
(int64) offset, amount));
/*
* Caution: do not call pg_flush_data with amount = 0, it could trash the
* file's seek position.
*/
if (amount <= 0)
return;
returnCode = FileAccess(file);
if (returnCode < 0)
return;
@ -2904,11 +2949,15 @@ pre_sync_fname(const char *fname, bool isdir, int elevel)
{
int fd;
/* Don't try to flush directories, it'll likely just fail */
if (isdir)
return;
fd = OpenTransientFile((char *) fname, O_RDONLY | PG_BINARY, 0);
if (fd < 0)
{
if (errno == EACCES || (isdir && errno == EISDIR))
if (errno == EACCES)
return;
ereport(elevel,
(errcode_for_file_access(),

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