From e1c345582c979ed44f881dec0b3137d862097c6b Mon Sep 17 00:00:00 2001 From: Chi Zhiling Date: Sat, 20 Jun 2026 14:24:45 +0800 Subject: [PATCH] mm/filemap: reduce unnecessary xarray lookups when read cached pages Patch series "mm/filemap: reduce unnecessary xarray lookups". This series optimizes xarray lookups in filemap by avoiding redundant iterations after obtaining the last needed folio. The boundary check is moved to before advancing the xarray iterator, eliminating unnecessary lookups and branches in the fast path. This reduces the overhead of filemap_get_read_batch() from 2.91% to 2.53% in 4k read tests. This patch (of 2): When reading small amounts of data from the page cache, only a single folio is typically returned from filemap_read_get_batch(). In this case, calling xas_advance() or xas_next() after adding the folio to the batch is unnecessary and only introduces extra branches. The same issue exists for large reads, where one additional xarray walk is always performed before termination. Quit the loop once we get the last folio in the range, so the final redundant xarray advancement can be avoided. The xas_next() does not update xa_index when xas->xa_node is set to XAS_RESTART, so the put and retry path would not update xa_index, hence the warning should therefore never trigger. During the 4k reads test, the overhead of this function dropped from 2.91% to 2.53%. Link: https://lore.kernel.org/20260620062446.351475-2-chizhiling@163.com Signed-off-by: Chi Zhiling Suggested-by: Matthew Wilcox (Oracle) Reviewed-by: Jan Kara Cc: Chi Zhiling Cc: Matthew Wilcox (Oracle) Signed-off-by: Andrew Morton --- mm/filemap.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/mm/filemap.c b/mm/filemap.c index 58eb9d240643..dfc22df1031a 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2467,11 +2467,14 @@ static void filemap_get_read_batch(struct address_space *mapping, XA_STATE(xas, &mapping->i_pages, index); struct folio *folio; + if (index > max) + return; + rcu_read_lock(); for (folio = xas_load(&xas); folio; folio = xas_next(&xas)) { if (xas_retry(&xas, folio)) continue; - if (xas.xa_index > max || xa_is_value(folio)) + if (xa_is_value(folio)) break; if (xa_is_sibling(folio)) break; @@ -2488,6 +2491,8 @@ static void filemap_get_read_batch(struct address_space *mapping, if (folio_test_readahead(folio)) break; xas_advance(&xas, folio_next_index(folio) - 1); + if (xas.xa_index >= max) + break; continue; put_folio: folio_put(folio);