From 4621c7f7a432fbb24b69c5cf0c877c17832c26ac Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Mon, 25 Sep 2017 16:09:19 -0400
Subject: [PATCH] Avoid SIGBUS on Linux when a DSM memory request overruns
tmpfs.
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
---
configure | 2 +-
configure.in | 2 +-
src/backend/storage/ipc/dsm_impl.c | 54 ++++++++++++++++++++++++++++--
src/include/pg_config.h.in | 3 ++
src/include/pg_config.h.win32 | 3 ++
5 files changed, 60 insertions(+), 4 deletions(-)
diff --git a/configure b/configure
index 84faf6af3ee..85cf0e1b1d4 100755
--- a/configure
+++ b/configure
@@ -13024,7 +13024,7 @@ fi
LIBS_including_readline="$LIBS"
LIBS=`echo "$LIBS" | sed -e 's/-ledit//g' -e 's/-lreadline//g'`
-for ac_func in cbrt clock_gettime dlopen fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll pstat pthread_is_threaded_np readlink setproctitle setsid shm_open symlink sync_file_range towlower utime utimes wcstombs wcstombs_l
+for ac_func in cbrt clock_gettime dlopen fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll posix_fallocate pstat pthread_is_threaded_np readlink setproctitle setsid shm_open symlink sync_file_range towlower utime utimes wcstombs wcstombs_l
do :
as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"
diff --git a/configure.in b/configure.in
index f6fcacc89c9..aa7d67165d8 100644
--- a/configure.in
+++ b/configure.in
@@ -1430,7 +1430,7 @@ PGAC_FUNC_WCSTOMBS_L
LIBS_including_readline="$LIBS"
LIBS=`echo "$LIBS" | sed -e 's/-ledit//g' -e 's/-lreadline//g'`
-AC_CHECK_FUNCS([cbrt clock_gettime dlopen fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll pstat pthread_is_threaded_np readlink setproctitle setsid shm_open symlink sync_file_range towlower utime utimes wcstombs wcstombs_l])
+AC_CHECK_FUNCS([cbrt clock_gettime dlopen fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll posix_fallocate pstat pthread_is_threaded_np readlink setproctitle setsid shm_open symlink sync_file_range towlower utime utimes wcstombs wcstombs_l])
AC_REPLACE_FUNCS(fseeko)
case $host_os in
diff --git a/src/backend/storage/ipc/dsm_impl.c b/src/backend/storage/ipc/dsm_impl.c
index 1500465d31c..4c1addc77c5 100644
--- a/src/backend/storage/ipc/dsm_impl.c
+++ b/src/backend/storage/ipc/dsm_impl.c
@@ -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
/*
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 579d195663c..c65dd7db215 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -393,6 +393,9 @@
/* Define to 1 if you have the `posix_fadvise' function. */
#undef HAVE_POSIX_FADVISE
+/* Define to 1 if you have the `posix_fallocate' function. */
+#undef HAVE_POSIX_FALLOCATE
+
/* Define to 1 if the assembler supports PPC's LWARX mutex hint bit. */
#undef HAVE_PPC_LWARX_MUTEX_HINT
diff --git a/src/include/pg_config.h.win32 b/src/include/pg_config.h.win32
index 91db0ddd38c..a0bdcbda4eb 100644
--- a/src/include/pg_config.h.win32
+++ b/src/include/pg_config.h.win32
@@ -261,6 +261,9 @@
/* Define to 1 if you have the <poll.h> header file. */
/* #undef HAVE_POLL_H */
+/* Define to 1 if you have the `posix_fallocate' function. */
+/* #undef HAVE_POSIX_FALLOCATE */
+
/* Define to 1 if you have the `pstat' function. */
/* #undef HAVE_PSTAT */
--
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