Re: Hang with v4.15-rc trying to swap back in

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 17 Jan 2018 14:33:21 -0800 (PST) Hugh Dickins <hughd@google.com> wrote:

> On Thu, 28 Dec 2017, James Bottomley wrote:
> > On Thu, 2017-12-28 at 09:41 -0800, James Bottomley wrote:
> > > I'd guess that since they're both in io_schedule, the problem is that
> > > the io_scheduler is taking far too long servicing the requests due to
> > > some priority issue you've introduced.
> > 
> > OK, so after some analysis, that turned out to be incorrect.  The
> > problem seems to be that we're exiting do_swap_page() with locked pages
> > that have been read in from swap.
> > 
> > Your changelogs are entirely unclear on why you changed the swapcache
> > setting logic in this patch:
> > 
> > commit 0bcac06f27d7528591c27ac2b093ccd71c5d0168
> > Author: Minchan Kim <minchan@kernel.org>
> > Date:   Wed Nov 15 17:33:07 2017 -0800
> > 
> >     mm, swap: skip swapcache for swapin of synchronous device
> > 
> > But I think you're using swapcache == NULL as a signal the page came
> > from a synchronous device.  In which case the bug is that you've
> > forgotten we may already have picked up a page in
> > swap_readahead_detect() which you're wrongly keeping swapcache == NULL
> > for and the fix is this (it works on my system, although I'm still
> > getting an unaccountable shutdown delay).
> > 
> > I still think we should revert this series, because this may not be the
> > only bug lurking in the code, so it should go through a lot more
> > rigorous testing than it has.
> 
> Andrew, neither the fix below (works for me, though I have seen other
> swap funniness, most probably unrelated), nor the reversion preferred
> by James and Minchan (later in this linux-mm thread), was in 4.15-rc8:
> the sands of time are running out...

Yup.  I'm actually planning on sending in this one.  OK by you?


From: Minchan Kim <minchan@kernel.org>
Subject: mm/memory.c: release locked page in do_swap_page()

James reported a bug in swap paging-in from his testing.  It is that
do_swap_page doesn't release locked page so system hang-up happens due to
a deadlock on PG_locked.

It was introduced by 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of
synchronous device") because I missed swap cache hit places to update
swapcache variable to work well with other logics against swapcache in
do_swap_page.

This patch fixes it.

Debugged by James Bottomley.

Link: http://lkml.kernel.org/r/<1514407817.4169.4.camel@HansenPartnership.com>
Link: http://lkml.kernel.org/r/20180102235606.GA19438@bbox
Signed-off-by: Minchan Kim <minchan@kernel.org>
Reported-by: James Bottomley <James.Bottomley@hansenpartnership.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memory.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff -puN mm/memory.c~mm-release-locked-page-in-do_swap_page mm/memory.c
--- a/mm/memory.c~mm-release-locked-page-in-do_swap_page
+++ a/mm/memory.c
@@ -2857,8 +2857,11 @@ int do_swap_page(struct vm_fault *vmf)
 	int ret = 0;
 	bool vma_readahead = swap_use_vma_readahead();
 
-	if (vma_readahead)
+	if (vma_readahead) {
 		page = swap_readahead_detect(vmf, &swap_ra);
+		swapcache = page;
+	}
+
 	if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte)) {
 		if (page)
 			put_page(page);
@@ -2889,9 +2892,12 @@ int do_swap_page(struct vm_fault *vmf)
 
 
 	delayacct_set_flag(DELAYACCT_PF_SWAPIN);
-	if (!page)
+	if (!page) {
 		page = lookup_swap_cache(entry, vma_readahead ? vma : NULL,
 					 vmf->address);
+		swapcache = page;
+	}
+
 	if (!page) {
 		struct swap_info_struct *si = swp_swap_info(entry);
 
_

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