readahead: don't shorten readahead window in read_pages()

commit 7a1eb89
Author: Jan Kara <jack@suse.cz>
Date:   Wed Dec 4 19:10:15 2024 +0100

    readahead: don't shorten readahead window in read_pages()

    Patch series "readahead: Reintroduce fix for improper RA window sizing".

    This small patch series reintroduces a fix of readahead window confusion
    (and thus read throughput reduction) when page_cache_ra_order() ends up
    failing due to folios already present in the page cache.  After thinking
    about this for a while I have ended up with a dumb fix that just rechecks
    if we have something to read before calling do_page_cache_ra().  This
    fixes the problem reported in [1].  I still think it doesn't make much
    sense to update readahead window size in read_pages() so patch 1 removes
    that but the real fix in patch 2 does not depend on it.

    [1] https://lore.kernel.org/all/49648605-d800-4859-be49-624bbe60519d@gmail.com

    This patch (of 2):

    When ->readahead callback doesn't read all requested pages, read_pages()
    shortens the readahead window (ra->size).  However we don't know why pages
    were not read and what appropriate window size is.  So don't try to
    secondguess the filesystem.  If it needs different readahead window, it
    should set it manually similarly as during expansion the filesystem can
    use readahead_expand().

    Link: https://lkml.kernel.org/r/20241204181016.15273-1-jack@suse.cz
    Link: https://lkml.kernel.org/r/20241204181016.15273-2-jack@suse.cz
    Signed-off-by: Jan Kara <jack@suse.cz>
    Cc: Matthew Wilcox <willy@infradead.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

JIRA: https://issues.redhat.com/browse/RHEL-77742
Signed-off-by: Nico Pache <npache@redhat.com>

Author: PacheNico

mm/readahead.c

@@ -158,20 +158,10 @@ static void read_pages(struct readahead_control *rac)
 
 	if (aops->readahead) {
 		aops->readahead(rac);
-		/*
-		 * Clean up the remaining folios.  The sizes in ->ra
-		 * may be used to size the next readahead, so make sure
-		 * they accurately reflect what happened.
-		 */
+		/* Clean up the remaining folios. */
 		while ((folio = readahead_folio(rac)) != NULL) {
-			unsigned long nr = folio_nr_pages(folio);
-
 			folio_get(folio);
-			rac->ra->size -= nr;
-			if (rac->ra->async_size >= nr) {
-				rac->ra->async_size -= nr;
-				filemap_remove_folio(folio);
-			}
+			filemap_remove_folio(folio);
 			folio_unlock(folio);
 			folio_put(folio);
 		}