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    SubjectRe: [PATCH 6/9] mm/rmap: use rmap_walk() in try_to_unmap()
    On Thu, 28 Nov 2013 16:48:43 +0900 Joonsoo Kim <iamjoonsoo.kim@lge.com> wrote:

    > Now, we have an infrastructure in rmap_walk() to handle difference
    > from variants of rmap traversing functions.
    >
    > So, just use it in try_to_unmap().
    >
    > In this patch, I change following things.
    >
    > 1. enable rmap_walk() if !CONFIG_MIGRATION.
    > 2. mechanical change to use rmap_walk() in try_to_unmap().
    >
    > ...
    >
    > --- a/include/linux/rmap.h
    > +++ b/include/linux/rmap.h
    > @@ -190,7 +190,7 @@ int page_referenced_one(struct page *, struct vm_area_struct *,
    >
    > int try_to_unmap(struct page *, enum ttu_flags flags);
    > int try_to_unmap_one(struct page *, struct vm_area_struct *,
    > - unsigned long address, enum ttu_flags flags);
    > + unsigned long address, void *arg);

    This change is ugly and unchangelogged.

    Also, "enum ttu_flags flags" was nice and meaningful, but "void *arg"
    conveys far less information. A suitable way to address this
    shortcoming is to document `arg' at the try_to_unmap_one() definition
    site. try_to_unmap_one() doesn't actually have any documentation at
    this stage - let's please fix that?

    >
    > ...
    >
    > @@ -1509,6 +1510,11 @@ bool is_vma_temporary_stack(struct vm_area_struct *vma)
    > return false;
    > }
    >
    > +static int skip_vma_temporary_stack(struct vm_area_struct *vma, void *arg)
    > +{
    > + return (int)is_vma_temporary_stack(vma);
    > +}

    The (int) cast is unneeded - the compiler will turn a bool into an int.

    Should this function (and rmap_walk_control.skip()) really be returning
    a bool?

    The name of this function is poor: "skip_foo" implies that the function
    will skip over a foo. But that isn't what this function does. Please
    choose something which accurately reflects the function's behavior.

    > /**
    > * try_to_unmap_anon - unmap or unlock anonymous page using the object-based
    > * rmap method
    > @@ -1554,7 +1560,7 @@ static int try_to_unmap_anon(struct page *page, enum ttu_flags flags)
    > continue;
    >
    > address = vma_address(page, vma);
    > - ret = try_to_unmap_one(page, vma, address, flags);
    > + ret = try_to_unmap_one(page, vma, address, (void *)flags);
    > if (ret != SWAP_AGAIN || !page_mapped(page))
    > break;
    > }
    >
    > ...
    >
    > /**
    > * try_to_unmap - try to remove all page table mappings to a page
    > * @page: the page to get unmapped
    > @@ -1630,16 +1641,30 @@ out:
    > int try_to_unmap(struct page *page, enum ttu_flags flags)
    > {
    > int ret;
    > + struct rmap_walk_control rwc;
    >
    > - BUG_ON(!PageLocked(page));
    > VM_BUG_ON(!PageHuge(page) && PageTransHuge(page));
    >
    > - if (unlikely(PageKsm(page)))
    > - ret = try_to_unmap_ksm(page, flags);
    > - else if (PageAnon(page))
    > - ret = try_to_unmap_anon(page, flags);
    > - else
    > - ret = try_to_unmap_file(page, flags);
    > + memset(&rwc, 0, sizeof(rwc));
    > + rwc.main = try_to_unmap_one;
    > + rwc.arg = (void *)flags;
    > + rwc.main_done = page_not_mapped;
    > + rwc.file_nonlinear = try_to_unmap_nonlinear;
    > + rwc.anon_lock = page_lock_anon_vma_read;

    struct rmap_walk_control rwc = {
    ...
    };

    > + /*
    > + * During exec, a temporary VMA is setup and later moved.
    > + * The VMA is moved under the anon_vma lock but not the
    > + * page tables leading to a race where migration cannot
    > + * find the migration ptes. Rather than increasing the
    > + * locking requirements of exec(), migration skips
    > + * temporary VMAs until after exec() completes.
    > + */
    > + if (flags & TTU_MIGRATION && !PageKsm(page) && PageAnon(page))
    > + rwc.vma_skip = skip_vma_temporary_stack;
    > +
    > + ret = rmap_walk(page, &rwc);
    > +
    > if (ret != SWAP_MLOCK && !page_mapped(page))
    > ret = SWAP_SUCCESS;
    > return ret;
    >
    > ...
    >



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