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    SubjectRe: [PATCH 1/4] mm: Trial do_wp_page() simplification
    On Wed, Sep 16, 2020 at 02:46:19PM -0400, Peter Xu wrote:

    > My understanding is this may only work for the case when the fork()ed child
    > quitted before we reach here (so we still have mapcount==1 for the
    > page).

    Yes

    > What if not? Then mapcount will be greater than 1, and cow will
    > still trigger. Is that what we want?

    That doesn't work today anyhow, so it is fine continuing to be broken.

    > Another problem is that, aiui, one of the major change previous patch proposed
    > is to avoid using lock_page() so that we never block in this path.

    I saw you mention this before, but it looks like the change was to
    lift some of the atomc_reads out of the lock and avoid the lock if
    they indicate failure, checking also for page_maybe_dma_pinned()
    outside the lock just means the rare case of FOLL_PIN we will take the
    lock again.

    > Maybe even more complicated, because "correctness" should be even harder
    > than "best effort reuse" since it can cause data corruption if we didn't do it
    > right...

    The only correct way is for the application to avoid write protect on
    FOLL_PIN pages. The purpose here is to allow applications that hadn't
    hit "bad luck" and failed to keep working.

    Another thought is to insert a warning print here as well that the
    program is working improperly? At least it would give a transition
    period to evaluate the extent of the problem.

    We are thinking it is going to be a notable regression.

    I botched the last version of the patch, here is something a bit
    better.

    Does it seem like it could be OK? I know very little about this part
    of the kernel

    Thanks,
    Jason

    diff --git a/mm/memory.c b/mm/memory.c
    index 469af373ae76e1..332de777854f8b 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -2889,6 +2889,24 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
    return ret;
    }

    +static bool cow_needed(struct vm_fault *vmf)
    +{
    + int total_map_swapcount;
    +
    + if (!reuse_swap_page(vmf->page, &total_map_swapcount))
    + return true;
    +
    + if (total_map_swapcount == 1) {
    + /*
    + * The page is all ours. Move it to our anon_vma so the rmap
    + * code will not search our parent or siblings. Protected
    + * against the rmap code by the page lock.
    + */
    + page_move_anon_rmap(vmf->page, vmf->vma);
    + }
    + return false;
    +}
    +
    /*
    * This routine handles present pages, when users try to write
    * to a shared page. It is done by copying the page to a new address
    @@ -2942,13 +2960,27 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
    struct page *page = vmf->page;

    /* PageKsm() doesn't necessarily raise the page refcount */
    - if (PageKsm(page) || page_count(page) != 1)
    + if (PageKsm(page))
    goto copy;
    + if (page_count(page) != 1) {
    + /*
    + * If the page is DMA pinned we can't rely on the
    + * above to know if there are other CPU references as
    + * page_count() will be elevated by the
    + * pin. Needlessly copying the page will cause the DMA
    + * pin to break, try harder to avoid that.
    + */
    + if (!page_maybe_dma_pinned(page))
    + goto copy;
    + }
    +
    if (!trylock_page(page))
    goto copy;
    if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
    - unlock_page(page);
    - goto copy;
    + if (cow_needed(vmf)) {
    + unlock_page(page);
    + goto copy;
    + }
    }
    /*
    * Ok, we've got the only map reference, and the only
    \
     
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      Last update: 2020-09-17 13:27    [W:4.799 / U:0.072 seconds]
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