Filesystem write pages going directly to Active

Filesystem write pages going directly to Active

[Buddy Lumpkin]

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Hi Folks,

It is my understanding that pages always begin life on the Inactive LRU list and are 
only promoted to the Active list after they have been referenced a second time. It was 
recently called to my attention that this is not the case for writes.

I checked and sure enough, filesystem writes go straight to Active in the 4.x kernel
until that behavior was “fixed” in 4.7 (see commit below). As far as I can tell, this
bug was fixed on accident.

I tested kernels going back to 3.16.53, and they all have this behavior. I just want
to make sure ... does everyone agree this was a bug?

—Buddy



[buddy@buddy-test linux]$ git log -p -1 bbddabe2e436aa7869b3ac5248df5c14ddde0cbf
commit bbddabe2e436aa7869b3ac5248df5c14ddde0cbf
Author: Johannes Weiner <hannes@cmpxchg.org>
Date:   Fri May 20 16:56:28 2016 -0700

    mm: filemap: only do access activations on reads
    
    Andres observed that his database workload is struggling with the
    transaction journal creating pressure on frequently read pages.
    
    Access patterns like transaction journals frequently write the same
    pages over and over, but in the majority of cases those pages are never
    read back.  There are no caching benefits to be had for those pages, so
    activating them and having them put pressure on pages that do benefit
    from caching is a bad choice.
    
    Leave page activations to read accesses and don't promote pages based on
    writes alone.
    
    It could be said that partially written pages do contain cache-worthy
    data, because even if *userspace* does not access the unwritten part,
    the kernel still has to read it from the filesystem for correctness.
    However, a counter argument is that these pages enjoy at least *some*
    protection over other inactive file pages through the writeback cache,
    in the sense that dirty pages are written back with a delay and cache
    reclaim leaves them alone until they have been written back to disk.
    Should that turn out to be insufficient and we see increased read IO
    from partial writes under memory pressure, we can always go back and
    update grab_cache_page_write_begin() to take (pos, len) so that it can
    tell partial writes from pages that don't need partial reads.  But for
    now, keep it simple.
    
    Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
    Reported-by: Andres Freund <andres@anarazel.de>
    Cc: Rik van Riel <riel@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index beba6bd..8f48599 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2578,7 +2578,7 @@ struct page *grab_cache_page_write_begin(struct address_space *mapping,
                                        pgoff_t index, unsigned flags)
 {
        struct page *page;
-       int fgp_flags = FGP_LOCK|FGP_ACCESSED|FGP_WRITE|FGP_CREAT;
+       int fgp_flags = FGP_LOCK|FGP_WRITE|FGP_CREAT;
 
        if (flags & AOP_FLAG_NOFS)
                fgp_flags |= FGP_NOFS;


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