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

[Matthew Wilcox <willy@infradead.org>]

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.

        /*
         * XXX: GFP_NOFS allocations should rather fail than rely on
         * other request to make a forward progress.
         * We are in an unfortunate situation where out_of_memory cannot
         * do much for this context but let's try it to at least get
         * access to memory reserved if the current task is killed (see
         * out_of_memory). Once filesystems are ready to handle allocation
         * failures more gracefully we should just bail out here.
         */

What problem are you actually seeing?