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vmscan: convert the writeback handling in shrink_page_list() to folios 

Slightly more efficient due to fewer calls to compound_head().

Link: https://lkml.kernel.org/r/20220504182857.4013401-7-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Matthew Wilcox (Oracle) 2022-05-12 20:23:02 -07:00 committed by Andrew Morton
parent 1bee2c1677
commit d33e4e1412
1 changed files with 42 additions and 36 deletions
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78
mm/vmscan.c
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@ -1598,40 +1598,42 @@ retry:
stat->nr_congested += nr_pages;
/*
* If a page at the tail of the LRU is under writeback, there
* If a folio at the tail of the LRU is under writeback, there
* are three cases to consider.
*
* 1) If reclaim is encountering an excessive number of pages
* under writeback and this page is both under writeback and
* PageReclaim then it indicates that pages are being queued
* for IO but are being recycled through the LRU before the
* IO can complete. Waiting on the page itself risks an
* indefinite stall if it is impossible to writeback the
* page due to IO error or disconnected storage so instead
* note that the LRU is being scanned too quickly and the
* caller can stall after page list has been processed.
* 1) If reclaim is encountering an excessive number of folios
* under writeback and this folio is both under
* writeback and has the reclaim flag set then it
* indicates that folios are being queued for I/O but
* are being recycled through the LRU before the I/O
* can complete. Waiting on the folio itself risks an
* indefinite stall if it is impossible to writeback
* the folio due to I/O error or disconnected storage
* so instead note that the LRU is being scanned too
* quickly and the caller can stall after the folio
* list has been processed.
*
* 2) Global or new memcg reclaim encounters a page that is
* 2) Global or new memcg reclaim encounters a folio that is
* not marked for immediate reclaim, or the caller does not
* have __GFP_FS (or __GFP_IO if it's simply going to swap,
* not to fs). In this case mark the page for immediate
* not to fs). In this case mark the folio for immediate
* reclaim and continue scanning.
*
* Require may_enter_fs() because we would wait on fs, which
* may not have submitted IO yet. And the loop driver might
* enter reclaim, and deadlock if it waits on a page for
* may not have submitted I/O yet. And the loop driver might
* enter reclaim, and deadlock if it waits on a folio for
* which it is needed to do the write (loop masks off
* __GFP_IO|__GFP_FS for this reason); but more thought
* would probably show more reasons.
*
* 3) Legacy memcg encounters a page that is already marked
* PageReclaim. memcg does not have any dirty pages
* 3) Legacy memcg encounters a folio that already has the
* reclaim flag set. memcg does not have any dirty folio
* throttling so we could easily OOM just because too many
* pages are in writeback and there is nothing else to
* folios are in writeback and there is nothing else to
* reclaim. Wait for the writeback to complete.
*
* In cases 1) and 2) we activate the pages to get them out of
* the way while we continue scanning for clean pages on the
* In cases 1) and 2) we activate the folios to get them out of
* the way while we continue scanning for clean folios on the
* inactive list and refilling from the active list. The
* observation here is that waiting for disk writes is more
* expensive than potentially causing reloads down the line.
@ -1639,38 +1641,42 @@ retry:
* memory pressure on the cache working set any longer than it
* takes to write them to disk.
*/
if (PageWriteback(page)) {
if (folio_test_writeback(folio)) {
/* Case 1 above */
if (current_is_kswapd() &&
PageReclaim(page) &&
folio_test_reclaim(folio) &&
test_bit(PGDAT_WRITEBACK, &pgdat->flags)) {
stat->nr_immediate += nr_pages;
goto activate_locked;
/* Case 2 above */
} else if (writeback_throttling_sane(sc) ||
!PageReclaim(page) || !may_enter_fs(page, sc->gfp_mask)) {
!folio_test_reclaim(folio) ||
!may_enter_fs(page, sc->gfp_mask)) {
/*
* This is slightly racy - end_page_writeback()
* might have just cleared PageReclaim, then
* setting PageReclaim here end up interpreted
* as PageReadahead - but that does not matter
* enough to care. What we do want is for this
* page to have PageReclaim set next time memcg
* reclaim reaches the tests above, so it will
* then wait_on_page_writeback() to avoid OOM;
* and it's also appropriate in global reclaim.
* This is slightly racy -
* folio_end_writeback() might have just
* cleared the reclaim flag, then setting
* reclaim here ends up interpreted as
* the readahead flag - but that does
* not matter enough to care. What we
* do want is for this folio to have
* the reclaim flag set next time memcg
* reclaim reaches the tests above, so
* it will then folio_wait_writeback()
* to avoid OOM; and it's also appropriate
* in global reclaim.
*/
SetPageReclaim(page);
folio_set_reclaim(folio);
stat->nr_writeback += nr_pages;
goto activate_locked;
/* Case 3 above */
} else {
unlock_page(page);
wait_on_page_writeback(page);
/* then go back and try same page again */
list_add_tail(&page->lru, page_list);
folio_unlock(folio);
folio_wait_writeback(folio);
/* then go back and try same folio again */
list_add_tail(&folio->lru, page_list);
continue;
}
}