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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

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. A 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:A A A Wed Nov 15 17:33:07 2017 -0800

A A A A 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. A 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.

James

---

diff --git a/mm/memory.c b/mm/memory.c
index ca5674cbaff2..31f9845c340e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2847,7 +2847,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
 int do_swap_page(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
-	struct page *page = NULL, *swapcache = NULL;
+	struct page *page = NULL, *swapcache;
 	struct mem_cgroup *memcg;
 	struct vma_swap_readahead swap_ra;
 	swp_entry_t entry;
@@ -2892,6 +2892,7 @@ int do_swap_page(struct vm_fault *vmf)
 	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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