From: Hillf Danton <hdanton@sina.com>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Hillf Danton <hdanton@sina.com>,
syzbot <syzbot+7a0d9d0b26efefe61780@syzkaller.appspotmail.com>,
akpm@linux-foundation.org, arve@android.com,
christian@brauner.io, devel@driverdev.osuosl.org,
gregkh@linuxfoundation.org, hughd@google.com,
joel@joelfernandes.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, maco@android.com,
syzkaller-bugs@googlegroups.com, tkjos@android.com,
Markus Elfring <Markus.Elfring@web.de>
Subject: Re: possible deadlock in shmem_fallocate (4)
Date: Tue, 14 Jul 2020 13:32:05 +0800 [thread overview]
Message-ID: <20200714053205.15240-1-hdanton@sina.com> (raw)
In-Reply-To: <20200714034111.GD1696@sol.localdomain>
On Mon, 13 Jul 2020 20:41:11 -0700 Eric Biggers wrote:
> On Tue, Jul 14, 2020 at 11:32:52AM +0800, Hillf Danton wrote:
> >
> > Add FALLOC_FL_NOBLOCK and on the shmem side try to lock inode upon the
> > new flag. And the overall upside is to keep the current gfp either in
> > the khugepaged context or not.
> >
> > --- a/include/uapi/linux/falloc.h
> > +++ b/include/uapi/linux/falloc.h
> > @@ -77,4 +77,6 @@
> > */
> > #define FALLOC_FL_UNSHARE_RANGE 0x40
> >
> > +#define FALLOC_FL_NOBLOCK 0x80
> > +
>
> You can't add a new UAPI flag to fix a kernel-internal problem like this.
Sounds fair, see below.
What the report indicates is a missing PF_MEMALLOC_NOFS and it's
checked on the ashmem side and added as an exception before going
to filesystem. On shmem side, no more than a best effort is paid
on the inteded exception.
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -437,6 +437,7 @@ static unsigned long
ashmem_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
{
unsigned long freed = 0;
+ bool nofs;
/* We might recurse into filesystem code, so bail out if necessary */
if (!(sc->gfp_mask & __GFP_FS))
@@ -445,6 +446,11 @@ ashmem_shrink_scan(struct shrinker *shri
if (!mutex_trylock(&ashmem_mutex))
return -1;
+ /* enter filesystem with caution: nonblock on locking */
+ nofs = current->flags & PF_MEMALLOC_NOFS;
+ if (!nofs)
+ current->flags |= PF_MEMALLOC_NOFS;
+
while (!list_empty(&ashmem_lru_list)) {
struct ashmem_range *range =
list_first_entry(&ashmem_lru_list, typeof(*range), lru);
@@ -472,6 +478,8 @@ ashmem_shrink_scan(struct shrinker *shri
}
mutex_unlock(&ashmem_mutex);
out:
+ if (!nofs)
+ current->flags &= ~PF_MEMALLOC_NOFS;
return freed;
}
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2728,7 +2728,12 @@ static long shmem_fallocate(struct file
if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
return -EOPNOTSUPP;
- inode_lock(inode);
+ if (current->flags & PF_MEMALLOC_NOFS) {
+ /* this exception needs a best effort and no more */
+ if (!inode_trylock(inode))
+ return -EBUSY;
+ } else
+ inode_lock(inode);
if (mode & FALLOC_FL_PUNCH_HOLE) {
struct address_space *mapping = file->f_mapping;
next prev parent reply other threads:[~2020-07-14 5:32 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-26 21:25 possible deadlock in shmem_fallocate (4) syzbot
2020-03-07 21:43 ` [ashmem] " Eric Biggers
2020-03-08 15:05 ` Hillf Danton
2020-07-14 0:32 ` syzbot
2020-07-14 3:32 ` Hillf Danton
2020-07-14 3:41 ` Eric Biggers
2020-07-14 5:32 ` Hillf Danton [this message]
2020-07-14 8:26 ` Michal Hocko
2020-07-14 14:08 ` Hillf Danton
2020-07-14 14:18 ` Michal Hocko
2020-07-14 15:46 ` Todd Kjos
2020-07-14 16:41 ` Suren Baghdasaryan
2020-07-14 17:32 ` Suren Baghdasaryan
2020-07-15 3:52 ` Hillf Danton
2020-07-15 6:36 ` Michal Hocko
2020-07-16 2:49 ` Suren Baghdasaryan
2020-07-14 3:07 ` syzbot
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