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Avoid SIGBUS on Linux when a DSM memory request overruns tmpfs.

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On Linux, shared memory segments created with shm_open() are backed by
swap files created in tmpfs.  If the swap file needs to be extended,
but there's no tmpfs space left, you get a very unfriendly SIGBUS trap.
To avoid this, force allocation of the full request size when we create
the segment.  This adds a few cycles, but none that we wouldn't expend
later anyway, assuming the request isn't hugely bigger than the actual
need.

Make this code #ifdef __linux__, because (a) there's not currently a
reason to think the same problem exists on other platforms, and (b)
applying posix_fallocate() to an FD created by shm_open() isn't very
portable anyway.

Back-patch to 9.4 where the DSM code came in.

Thomas Munro, per a bug report from Amul Sul

Discussion: https://postgr.es/m/1002664500.12301802.1471008223422.JavaMail.yahoo@mail.yahoo.com
This commit is contained in:
Tom Lane
2017-09-25 16:09:19 -04:00
parent 716ea626a8
commit 899bd785c0
5 changed files with 60 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files

54
src/backend/storage/ipc/dsm_impl.c
View File

@ -73,6 +73,7 @@
static bool dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size,
void **impl_private, void **mapped_address,
Size *mapped_size, int elevel);
static int dsm_impl_posix_resize(int fd, off_t size);
#endif
#ifdef USE_DSM_SYSV
static bool dsm_impl_sysv(dsm_op op, dsm_handle handle, Size request_size,
@ -319,7 +320,8 @@ dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size,
}
request_size = st.st_size;
}
else if (*mapped_size != request_size && ftruncate(fd, request_size))
else if (*mapped_size != request_size &&
dsm_impl_posix_resize(fd, request_size) != 0)
{
int save_errno;
@ -392,7 +394,55 @@ dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size,
return true;
}
#endif
/*
* Set the size of a virtual memory region associated with a file descriptor.
* If necessary, also ensure that virtual memory is actually allocated by the
* operating system, to avoid nasty surprises later.
*
* Returns non-zero if either truncation or allocation fails, and sets errno.
*/
static int
dsm_impl_posix_resize(int fd, off_t size)
{
int rc;
/* Truncate (or extend) the file to the requested size. */
rc = ftruncate(fd, size);
/*
* On Linux, a shm_open fd is backed by a tmpfs file. After resizing with
* ftruncate, the file may contain a hole. Accessing memory backed by a
* hole causes tmpfs to allocate pages, which fails with SIGBUS if there
* is no more tmpfs space available. So we ask tmpfs to allocate pages
* here, so we can fail gracefully with ENOSPC now rather than risking
* SIGBUS later.
*/
#if defined(HAVE_POSIX_FALLOCATE) && defined(__linux__)
if (rc == 0)
{
/* We may get interrupted, if so just retry. */
do
{
rc = posix_fallocate(fd, 0, size);
} while (rc == -1 && errno == EINTR);
if (rc != 0 && errno == ENOSYS)
{
/*
* Kernel too old (< 2.6.23). Rather than fail, just trust that
* we won't hit the problem (it typically doesn't show up without
* many-GB-sized requests, anyway).
*/
rc = 0;
}
}
#endif /* HAVE_POSIX_FALLOCATE && __linux__ */
return rc;
}
#endif /* USE_DSM_POSIX */
#ifdef USE_DSM_SYSV
/*