Re: [PATCH] squashfs: enable __GFP_FS in ->readpage to prevent hang in mem alloc

[Matthew Wilcox]

On Sun, Dec 16, 2018 at 05:38:13PM +0800, Hou Tao wrote:
> Hi,
> 
> On 2018/12/15 22:38, Matthew Wilcox wrote:
> > On Tue, Dec 04, 2018 at 10:08:40AM +0800, Hou Tao wrote:
> >> There is no need to disable __GFP_FS in ->readpage:
> >> * It's a read-only fs, so there will be no dirty/writeback page and
> >>   there will be no deadlock against the caller's locked page
> >> * It just allocates one page, so compaction will not be invoked
> >> * It doesn't take any inode lock, so the reclamation of inode will be fine
> >>
> >> And no __GFP_FS may lead to hang in __alloc_pages_slowpath() if a
> >> squashfs page fault occurs in the context of a memory hogger, because
> >> the hogger will not be killed due to the logic in __alloc_pages_may_oom().
> > 
> > I don't understand your argument here.  There's a comment in
> > __alloc_pages_may_oom() saying that we _should_ treat GFP_NOFS
> > specially, but we currently don't.
> I am trying to say that if __GFP_FS is used in pagecache_get_page() when it tries
> to allocate a new page for squashfs, that will be no possibility of dead-lock for
> squashfs.
> 
> We do treat GFP_NOFS specially in out_of_memory():
> 
>     /*
>      * The OOM killer does not compensate for IO-less reclaim.
>      * pagefault_out_of_memory lost its gfp context so we have to
>      * make sure exclude 0 mask - all other users should have at least
>      * ___GFP_DIRECT_RECLAIM to get here.
>      */
>     if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS))
>         return true;
> 
> So if GFP_FS is used, no task will be killed because we will return from
> out_of_memory() prematurely. And that will lead to an infinite loop in
> __alloc_pages_slowpath() as we have observed:
> 
> * a squashfs page fault occurred in the context of a memory hogger
> * the page used for page fault allocated successfully
> * in squashfs_readpage() squashfs will try to allocate other pages
>   in the same 128KB block, and __GFP_NOFS is used (actually GFP_HIGHUSER_MOVABLE & ~__GFP_FS)
> * in __alloc_pages_slowpath() we can not get any pages through reclamation
>   (because most of memory is used by the current task) and we also can not kill
>   the current task (due to __GFP_NOFS), and it will loop forever until it's killed.

Ah, yes, that makes perfect sense.  Thank you for the explanation.

I wonder if the correct fix, however, is not to move the check for
GFP_NOFS in out_of_memory() down to below the check whether to kill
the current task.  That would solve your problem, and I don't _think_
it would cause any new ones.  Michal, you touched this code last, what
do you think?