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    SubjectRe: [PATCH 11/11] mm: enlarge the "int nr_pages" parameter of update_lru_size()
    On Mon, 7 Dec 2020, Yu Zhao wrote:

    > update_lru_sizes() defines an unsigned long argument and passes it as
    > nr_pages to update_lru_size(). Though this isn't causing any overflows
    > I'm aware of, it's a bad idea to go through the demotion given that we
    > have recently stumbled on a related type promotion problem fixed by
    > commit 2da9f6305f30 ("mm/vmscan: fix NR_ISOLATED_FILE corruption on 64-bit")
    >
    > Note that the underlying counters are already in long. This is another
    > reason we shouldn't have the demotion.
    >
    > This patch enlarges all relevant parameters on the path to the final
    > underlying counters:
    > update_lru_size(int -> long)
    > if memcg:
    > __mod_lruvec_state(int -> long)
    > if smp:
    > __mod_node_page_state(long)
    > else:
    > __mod_node_page_state(int -> long)
    > __mod_memcg_lruvec_state(int -> long)
    > __mod_memcg_state(int -> long)
    > else:
    > __mod_lruvec_state(int -> long)
    > if smp:
    > __mod_node_page_state(long)
    > else:
    > __mod_node_page_state(int -> long)
    >
    > __mod_zone_page_state(long)
    >
    > if memcg:
    > mem_cgroup_update_lru_size(int -> long)
    >
    > Note that __mod_node_page_state() for the smp case and
    > __mod_zone_page_state() already use long. So this change also fixes
    > the inconsistency.
    >
    > Signed-off-by: Yu Zhao <yuzhao@google.com>

    NAK from me to this 11/11: I'm running happily with your 1-10 on top of
    mmotm (I'll review them n a few days, but currently more concerned with
    Rik's shmem huge gfp_mask), but had to leave this one out.

    You think you are future-proofing with this, but it is present-breaking.

    It looks plausible (though seems random: why these particular functions
    use long but others not? why __mod_memcg_state() long, mod_memcg_state()
    int?), and I was fooled; but fortunately was still testing with memcg
    moving, for Alex's patchset.

    Soon got stuck waiting in balance_dirty_pages(), /proc/vmstat showing
    nr_anon_pages 2263142822377729
    nr_mapped 125095217474159
    nr_file_pages 225421358649526
    nr_dirty 8589934592
    nr_writeback 1202590842920
    nr_shmem 40501541678768
    nr_anon_transparent_hugepages 51539607554

    That last (anon THPs) nothing to do with this patch, but illustrates
    what Muchun is fixing in his 1/7 "mm: memcontrol: fix NR_ANON_THPS
    accounting in charge moving".

    The rest of them could be fixed by changing mem_cgroup_move_account()'s
    "unsigned int nr_pages" to "long nr_pages" in this patch, but I think
    it's safer just to drop the patch: the promotion of "unsigned int" to
    "long" does not work as you would like it to.

    I see that mm/vmscan.c contains several "unsigned int" counts of pages,
    everything works fine at present so far as I know, and those appeared
    to work even with your patch; but I am not confident in my test coverage,
    and not confident in us being able to outlaw unsigned int page counts in
    future.

    Hugh

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