tmpfs: don't interrupt fallocate with EINTR

I have a program that sets up a periodic timer with 10ms interval. When
the program attempts to call fallocate(2) on tmpfs, it goes into an
infinite loop. fallocate(2) takes longer than 10ms, so it gets
interrupted by a signal and it returns EINTR. On EINTR, the fallocate
call is restarted, going into the same loop again.

Let's change the signal_pending() check in shmem_fallocate() loop to
fatal_signal_pending(). This solves the problem of shmem_fallocate()
constantly restarting. Since most other filesystem's fallocate methods
don't react to signals, it is unlikely userspace really relies on timely
delivery of non-fatal signals while fallocate is running. Also the
comment before the signal check:

/*
 * Good, the fallocate(2) manpage permits EINTR: we may have
 * been interrupted because we are using up too much memory.
 */

indicates that the check was mainly added for OOM situations in which
case the process will be sent a fatal signal so this change preserves
the behavior in OOM situations.

[JK: Update changelog and comment based on upstream discussion]

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20240515221044.590-1-jack@suse.cz
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index f5d60436b604af774856d2675b9bcb162a9aea95..ff7c756a7d022da90a12822d1e6d423b2462423c 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3166,10 +3166,13 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
                struct folio *folio;
 
                /*
-                * Good, the fallocate(2) manpage permits EINTR: we may have
-                * been interrupted because we are using up too much memory.
+                * Check for fatal signal so that we abort early in OOM
+                * situations. We don't want to abort in case of non-fatal
+                * signals as large fallocate can take noticeable time and
+                * e.g. periodic timers may result in fallocate constantly
+                * restarting.
                 */
-               if (signal_pending(current))
+               if (fatal_signal_pending(current))
                        error = -EINTR;
                else if (shmem_falloc.nr_unswapped > shmem_falloc.nr_falloced)
                        error = -ENOMEM;