On Mon, 12 Aug 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, hence this patch set supports the large folio swap out for shmem. > > Note the i915_gem_shmem driver still need to be split when swapping, thus > add a new flag 'split_large_folio' for writeback_control to indicate spliting > the large folio. Is that last paragraph a misunderstanding? The i915 THP splitting in shmem_writepage() was to avoid mm VM_BUG_ONs and crashes when shmem.c did not support huge page swapout: but now you are enabling that support, and such VM_BUG_ONs and crashes are gone (so far as I can see: and this is written on a laptop using the i915 driver). I cannot think of why i915 itself would care how mm implements swapout (beyond enjoying faster): I think all the wbc->split_large_folio you introduce here should be reverted. But you may know better! I do need a further change to shmem_writepage() here: see fixup patch below: that's written to apply on top of this 9/9, but I'd be glad to see a replacement with wbc->split_large_folio gone, and just one !IS_ENABLED(CONFIG_THP_SWAP) instead. > > [1] https://lore.kernel.org/all/cover.1717495894.git.baolin.wang@xxxxxxxxxxxxxxxxx/ > [2] https://lore.kernel.org/all/20240515055719.32577-1-da.gomez@xxxxxxxxxxx/ I get "Not found" for that [2] link. > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> > --- > drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 1 + > include/linux/writeback.h | 4 +++ > mm/shmem.c | 12 ++++++--- > mm/vmscan.c | 32 ++++++++++++++++++----- > 4 files changed, 38 insertions(+), 11 deletions(-) [PATCH] mm: shmem: shmem_writepage() split folio at EOF before swapout Working in a constrained (size= or nr_blocks=) huge=always tmpfs relies on swapout to split a large folio at EOF, to trim off its excess before hitting premature ENOSPC: shmem_unused_huge_shrink() contains no code to handle splitting huge swap blocks, and nobody would want that to be added. Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> --- mm/shmem.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/mm/shmem.c b/mm/shmem.c index 37c300f69baf..4dd0570962fa 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1459,6 +1459,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) swp_entry_t swap; pgoff_t index; int nr_pages; + bool split = false; /* * Our capabilities prevent regular writeback or sync from ever calling @@ -1480,8 +1481,20 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "always" or * "force", drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages, * and its shmem_writeback() needs them to be split when swapping. + * + * And shrinkage of pages beyond i_size does not split swap, so + * swapout of a large folio crossing i_size needs to split too + * (unless fallocate has been used to preallocate beyond EOF). */ - if (wbc->split_large_folio && folio_test_large(folio)) { + if (folio_test_large(folio)) { + split = wbc->split_large_folio; + index = shmem_fallocend(inode, + DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE)); + if (index > folio->index && index < folio_next_index(folio)) + split = true; + } + + if (split) { try_split: /* Ensure the subpages are still dirty */ folio_test_set_dirty(folio); -- 2.35.3
