Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation

From: David Hildenbrand
Date: Thu Nov 24 2022 - 09:10:50 EST


On 24.11.22 14:22, David Hildenbrand wrote:
On 24.11.22 13:55, Gavin Shan wrote:
On 11/24/22 6:43 PM, David Hildenbrand wrote:
On 24.11.22 11:21, Gavin Shan wrote:
On 11/24/22 6:09 PM, David Hildenbrand wrote:
On 24.11.22 10:55, Gavin Shan wrote:
The issue is reported when removing memory through virtio_mem device.
The transparent huge page, experienced copy-on-write fault, is wrongly
regarded as pinned. The transparent huge page is escaped from being
isolated in isolate_migratepages_block(). The transparent huge page
can't be migrated and the corresponding memory block can't be put
into offline state.

Fix it by replacing page_mapcount() with total_mapcount(). With this,
the transparent huge page can be isolated and migrated, and the memory
block can be put into offline state. Besides, The page's refcount is
increased a bit earlier to avoid the page is released when the check
is executed.

Did you look into handling pages that are in the swapcache case as well?

See is_refcount_suitable() in mm/khugepaged.c.

Should be easy to reproduce, let me know if you need inspiration.


Nope, I didn't look into the case. Please elaborate the details so that
I can reproduce it firstly.


A simple reproducer would be (on a system with ordinary swap (not zram))

1) mmap a region (MAP_ANON|MAP_PRIVATE) that can hold a THP

2) Enable THP for that region (MADV_HUGEPAGE)

3) Populate a THP (e.g., write access)

4) PTE-map the THP, for example, using MADV_FREE on the last subpage

5) Trigger swapout of the THP, for example, using MADV_PAGEOUT

6) Read-access to some subpages to fault them in from the swapcache


Now you'd have a THP, which

1) Is partially PTE-mapped into the page table
2) Is in the swapcache (each subpage should have one reference from the swapache)


Now we could test, if alloc_contig_range() will still succeed (e.g., using virtio-mem).


Thanks for the details. Step (4) and (5) can be actually combined. To swap part of
the THP (e.g. one sub-page) will force the THP to be split.

I followed your steps in the attached program, there is no issue to do memory hot-remove
through virtio-mem with or without this patch.

Interesting. But I don't really see how we could pass this check with a
page that's in the swapcache, maybe I'm missing something else.

I'll try to see if I can reproduce it.


After some unsuccessful attempts and many head-scratches, I realized that it's quite simple why we don't have to worry about swapcache pages here:

page_mapping() is != NULL for pages in the swapcache: folio_mapping() makes this rather obvious:

if (unlikely(folio_test_swapcache(folio))
return swap_address_space(folio_swap_entry(folio));


I think the get_page_unless_zero() might also be a fix for the page_mapping() call, smells like something could blow up on concurrent page freeing. (what about concurrent removal from the swapcache? nobody knows :) )


Thanks Gavin!

Acked-by: David Hildenbrand <david@xxxxxxxxxx>


--
Thanks,

David / dhildenb