Re: [PATCH] mm: fix long time stall from mm_populate

[Minchan Kim <minchan@kernel.org>]

On Wed, Feb 12, 2020 at 11:22:06AM +0100, Jan Kara wrote:
> On Mon 10-02-20 16:19:58, Minchan Kim wrote:
> > Basically, fault handler releases mmap_sem before requesting readahead
> > and then it is supposed to retry lookup the page from page cache with
> > FAULT_FLAG_TRIED so that it avoids the live lock of infinite retry.
> > 
> > However, what happens if the fault handler find a page from page
> > cache and the page has readahead marker but are waiting under
> > writeback? Plus one more condition, it happens under mm_populate
> > which repeats faulting unless it encounters error. So let's assemble
> > conditions below.
> > 
> > __mm_populate
> > for (...)
> >   get_user_pages(faluty_address)
> >     handle_mm_fault
> >       filemap_fault
> >         find a page form page(PG_uptodate|PG_readahead|PG_writeback)
> > 	it will return VM_FAULT_RETRY
> >   continue with faulty_address
> > 
> > IOW, it will repeat fault retry logic until the page will be written
> > back in the long run. It makes big spike latency of several seconds.
> > 
> > This patch solves the issue by turning off fault retry logic in second
> > trial.
> > 
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> > It was orignated from code review once I have seen several user reports
> > but didn't confirm yet it's the root cause.
> 
> Yes, I think the immediate problem is actually elsewhere but I agree that
> __mm_populate() should follow the general protocol of retrying only once
> so your change should make it more robust. The patch looks good to me, you
> can add:
> 
> Reviewed-by: Jan Kara <jack@suse.cz>

Thanks for the review!