Re: [PATCH 0/7] support large folio swap-out and swap-in for shmem

[Hugh Dickins]

On Thu, 6 Jun 2024, Baolin Wang wrote:

> Shmem will support large folio allocation [1] [2] to get a better performance,
> however, the memory reclaim still splits the precious large folios when trying
> to swap-out shmem, which may lead to the memory fragmentation issue and can not
> take advantage of the large folio for shmeme.
> 
> Moreover, the swap code already supports for swapping out large folio without
> split, and large folio swap-in[3] series is queued into mm-unstable branch.
> Hence this patch set also supports the large folio swap-out and swap-in for
> shmem.
> 
> [1] https://lore.kernel.org/all/cover.1717495894.git.baolin.wang@xxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/all/20240515055719.32577-1-da.gomez@xxxxxxxxxxx/
> [3] https://lore.kernel.org/all/20240508224040.190469-6-21cnbao@xxxxxxxxx/T/
> 
> Changes from RFC:
>  - Rebased to the latest mm-unstable.
>  - Drop the counter name fixing patch, which was queued into mm-hotfixes-stable
>  branch.
> 
> Baolin Wang (7):
>   mm: vmscan: add validation before spliting shmem large folio
>   mm: swap: extend swap_shmem_alloc() to support batch SWAP_MAP_SHMEM
>     flag setting
>   mm: shmem: support large folio allocation for shmem_replace_folio()
>   mm: shmem: extend shmem_partial_swap_usage() to support large folio
>     swap
>   mm: add new 'orders' parameter for find_get_entries() and
>     find_lock_entries()
>   mm: shmem: use swap_free_nr() to free shmem swap entries
>   mm: shmem: support large folio swap out
> 
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c |  1 +
>  include/linux/swap.h                      |  4 +-
>  include/linux/writeback.h                 |  1 +
>  mm/filemap.c                              | 27 ++++++-
>  mm/internal.h                             |  4 +-
>  mm/shmem.c                                | 58 ++++++++------
>  mm/swapfile.c                             | 98 ++++++++++++-----------
>  mm/truncate.c                             |  8 +-
>  mm/vmscan.c                               | 22 ++++-
>  9 files changed, 140 insertions(+), 83 deletions(-)

I wanted to have some tests running, while looking through these
and your shmem mTHP patches; but I wasted too much time on that by
applying these on top and hitting crash, OOMs and dreadful thrashing -
testing did not get very far at all.

Perhaps all easily fixed, but I don't have more time to spend on it,
and think this series cannot expect to go into 6.11: I'll have another
try with it next cycle.

I really must turn my attention to your shmem mTHP series: no doubt
I'll have minor adjustments to ask there - but several other people
are also waiting for me to respond (or given up on me completely).

The little crash fix needed in this series appears to be:

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2053,7 +2053,8 @@ static int shmem_swapin_folio(struct ino
 			goto failed;
 	}
 
-	error = shmem_add_to_page_cache(folio, mapping, index,
+	error = shmem_add_to_page_cache(folio, mapping,
+					round_down(index, nr_pages),
 					swp_to_radix_entry(swap), gfp);
 	if (error)
 		goto failed;

Then the OOMs and dreadful thrashing are due to refcount confusion:
I did not even glance at these patches to work out what's wanted,
but a printk in __remove_mapping() showed that folio->_refcount was
1024 where 513 was expected, so reclaim was freeing none of them.

Hugh