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SubjectRe: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]
On Thu, 1 Nov 2012, Dave Jones wrote:
> On Wed, Oct 24, 2012 at 09:36:27PM -0700, Hugh Dickins wrote:
> > On Wed, 24 Oct 2012, Dave Jones wrote:
> >
> > > Machine under significant load (4gb memory used, swap usage fluctuating)
> > > triggered this...
> > >
> > > WARNING: at mm/shmem.c:1151 shmem_getpage_gfp+0xa5c/0xa70()
> > >
> > > 1148 error = shmem_add_to_page_cache(page, mapping, index,
> > > 1149 gfp, swp_to_radix_entry(swap));
> > > 1150 /* We already confirmed swap, and make no allocation */
> > > 1151 VM_BUG_ON(error);
> > > 1152 }
> >
> > That's very surprising. Easy enough to handle an error there, but
> > of course I made it a VM_BUG_ON because it violates my assumptions:
> > I rather need to understand how this can be, and I've no idea.
>
> I just noticed we had a user report hitting this same warning, but
> with a different trace..
>
> : [<ffffffff8105b84f>] warn_slowpath_common+0x7f/0xc0
> : [<ffffffff8105b8aa>] warn_slowpath_null+0x1a/0x20
> : [<ffffffff81143c73>] shmem_getpage_gfp+0x7f3/0x830
> : [<ffffffff81158c9d>] ? vma_adjust+0x3ed/0x620
> : [<ffffffff81143f02>] shmem_file_aio_read+0x1f2/0x380
> : [<ffffffff8118e487>] do_sync_read+0xa7/0xe0
> : [<ffffffff8118eda9>] vfs_read+0xa9/0x180
> : [<ffffffff8118eeca>] sys_read+0x4a/0x90
> : [<ffffffff816226e9>] system_call_fastpath+0x16/0x1b

Equally explicable by Hannes's hypothesis;
but useful supporting evidence, thank you.

Except... earlier in the thread you explained how you hacked
#define VM_BUG_ON(cond) WARN_ON(cond)
to get this to come out as a warning instead of a bug,
and now it looks as if "a user" has here done the same.

Which is very much a user's right, of course; but does
make me wonder whether that user might actually be davej ;)

Never mind, whatever, it's more justification for the fix - which
I've honestly not forgotten, but somehow not got around to sending
(with a couple of others even longer outstanding). On its way
shortly, for some unpredictable value of shortly.

Hugh


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