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Date:   Wed, 6 Nov 2019 15:38:55 +0100
From:   Jan Kara <jack@suse.cz>
To:     snazy@snazy.de
Cc:     Jan Kara <jack@suse.cz>, Johannes Weiner <hannes@cmpxchg.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>,
        Josef Bacik <josef@toxicpanda.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Randy Dunlap <rdunlap@infradead.org>,
        linux-kernel@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Potyra, Stefan" <Stefan.Potyra@elektrobit.com>
Subject: Re: mlockall(MCL_CURRENT) blocking infinitely
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On Wed 06-11-19 14:45:43, Robert Stupp wrote:
> On Wed, 2019-11-06 at 13:03 +0100, Jan Kara wrote:
> > On Tue 05-11-19 13:22:11, Johannes Weiner wrote:
> > > What I don't quite understand yet is why the fault path doesn't
> > > make
> > > progress eventually. We must drop the mmap_sem without changing the
> > > state in any way. How can we keep looping on the same page?
> >
> > That may be a slight suboptimality with Josef's patches. If the page
> > is marked as PageReadahead, we always drop mmap_sem if we can and
> > start
> > readahead without checking whether that makes sense or not in
> > do_async_mmap_readahead(). OTOH page_cache_async_readahead() then
> > clears
> > PageReadahead so the only way how I can see we could loop like this
> > is when
> > file->ra->ra_pages is 0. Not sure if that's what's happening through.
> > We'd
> > need to find which of the paths in filemap_fault() calls
> > maybe_unlock_mmap_for_io() to tell more.
> 
> Yes, ra_pages==0

BTW, attached patch should workaround your problem as well. But that's just
a performance optimization that happens to paper over your problem. Real
fix is the proper handling of fault retry as you did it.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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From e12c861e687364ff5a891f0ae90283b384d74197 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Wed, 6 Nov 2019 15:30:26 +0100
Subject: [PATCH] mm: Don't bother dropping mmap_sem for zero size readahead

When handling a page fault, we drop mmap_sem to start async readahead so
that we don't block on IO submission with mmap_sem held.  However
there's no point to drop mmap_sem in case readahead is disabled. Handle
that case to avoid pointless dropping of mmap_sem and retrying the
fault.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 mm/filemap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 1146fcfa3215..3d39c437b07e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2458,7 +2458,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
 	pgoff_t offset = vmf->pgoff;
 
 	/* If we don't want any read-ahead, don't bother */
-	if (vmf->vma->vm_flags & VM_RAND_READ)
+	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
 		return fpin;
 	if (ra->mmap_miss > 0)
 		ra->mmap_miss--;
-- 
2.16.4


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