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    SubjectRe: [PATCH 2/6] mm: mlocking in try_to_unmap_one
    On Wed, 11 Nov 2009, KOSAKI Motohiro wrote:

    Though it doesn't quite answer your question,
    I'll just reinsert the last paragraph of my description here...

    > > try_to_unmap_file()'s TTU_MUNLOCK nonlinear handling was particularly
    > > amusing: once unravelled, it turns out to have been choosing between
    > > two different ways of doing the same nothing. Ah, no, one way was
    > > actually returning SWAP_FAIL when it meant to return SWAP_SUCCESS.

    ...
    > > @@ -1081,45 +1053,23 @@ static int try_to_unmap_file(struct page
    ...
    > >
    > > - if (list_empty(&mapping->i_mmap_nonlinear))
    > > + /* We don't bother to try to find the munlocked page in nonlinears */
    > > + if (MLOCK_PAGES && TTU_ACTION(flags) == TTU_MUNLOCK)
    > > goto out;
    >
    > I have dumb question.
    > Does this shortcut exiting code makes any behavior change?

    Not dumb. My intention was to make no behaviour change with any of
    this patch; but in checking back before completing the description,
    I suddenly realized that that shortcut intentionally avoids the

    if (max_nl_size == 0) { /* all nonlinears locked or reserved ? */
    ret = SWAP_FAIL;
    goto out;
    }

    (which doesn't show up in the patch: you'll have to look at rmap.c),
    which used to have the effect of try_to_munlock() returning SWAP_FAIL
    in the case when there were one or more VM_NONLINEAR vmas of the file,
    but none of them (and none of the covering linear vmas) VM_LOCKED.

    That should have been a SWAP_SUCCESS case, or with my changes
    another SWAP_AGAIN, either of which would make munlock_vma_page()
    count_vm_event(UNEVICTABLE_PGMUNLOCKED);
    which would be correct; but the SWAP_FAIL meant that count was not
    incremented in this case.

    Actually, I've double-fixed that, because I also changed
    munlock_vma_page() to increment the count whenever ret != SWAP_MLOCK;
    which seemed more appropriate, but would have been a no-op if
    try_to_munlock() only returned SWAP_SUCCESS or SWAP_AGAIN or SWAP_MLOCK
    as it claimed.

    But I wasn't very inclined to boast of fixing that bug, since my testing
    didn't give confidence that those /proc/vmstat unevictable_pgs_*lock*
    counts are being properly maintained anyway - when I locked the same
    pages in two vmas then unlocked them in both, I ended up with mlocked
    bigger than munlocked (with or without my 2/6 patch); which I suspect
    is wrong, but rather off my present course towards KSM swapping...

    Hugh


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