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    SubjectRe: [PATCH 3/3] mm, thp: make swapin readahead under down_read of mmap_sem
    On Mon, May 23, 2016 at 08:14:11PM +0300, Ebru Akagunduz wrote:
    > Currently khugepaged makes swapin readahead under
    > down_write. This patch supplies to make swapin
    > readahead under down_read instead of down_write.
    >
    > The patch was tested with a test program that allocates
    > 800MB of memory, writes to it, and then sleeps. The system
    > was forced to swap out all. Afterwards, the test program
    > touches the area by writing, it skips a page in each
    > 20 pages of the area.
    >
    > Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
    > ---
    > mm/huge_memory.c | 33 +++++++++++++++++++++++++++------
    > 1 file changed, 27 insertions(+), 6 deletions(-)
    >
    > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    > index feee44c..668bc07 100644
    > --- a/mm/huge_memory.c
    > +++ b/mm/huge_memory.c
    > @@ -2386,13 +2386,14 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
    > * but with mmap_sem held to protect against vma changes.
    > */
    >
    > -static void __collapse_huge_page_swapin(struct mm_struct *mm,
    > +static bool __collapse_huge_page_swapin(struct mm_struct *mm,
    > struct vm_area_struct *vma,
    > unsigned long address, pmd_t *pmd)
    > {
    > unsigned long _address;
    > pte_t *pte, pteval;
    > int swapped_in = 0, ret = 0;
    > + struct vm_area_struct *vma_orig = vma;
    >
    > pte = pte_offset_map(pmd, address);
    > for (_address = address; _address < address + HPAGE_PMD_NR*PAGE_SIZE;
    > @@ -2402,11 +2403,19 @@ static void __collapse_huge_page_swapin(struct mm_struct *mm,
    > continue;
    > swapped_in++;
    > ret = do_swap_page(mm, vma, _address, pte, pmd,
    > - FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT,
    > + FAULT_FLAG_ALLOW_RETRY,
    > pteval);
    > + /* do_swap_page returns VM_FAULT_RETRY with released mmap_sem */
    > + if (ret & VM_FAULT_RETRY) {
    > + down_read(&mm->mmap_sem);
    > + vma = find_vma(mm, address);
    > + /* vma is no longer available, don't continue to swapin */
    > + if (vma != vma_orig)
    > + return false;
    > + }
    > if (ret & VM_FAULT_ERROR) {
    > trace_mm_collapse_huge_page_swapin(mm, swapped_in, 0);
    > - return;
    > + return false;
    > }
    > /* pte is unmapped now, we need to map it */
    > pte = pte_offset_map(pmd, _address);
    > @@ -2414,6 +2423,7 @@ static void __collapse_huge_page_swapin(struct mm_struct *mm,
    > pte--;
    > pte_unmap(pte);
    > trace_mm_collapse_huge_page_swapin(mm, swapped_in, 1);
    > + return true;
    > }
    >
    > static void collapse_huge_page(struct mm_struct *mm,
    > @@ -2459,7 +2469,7 @@ static void collapse_huge_page(struct mm_struct *mm,
    > * gup_fast later hanlded by the ptep_clear_flush and the VM
    > * handled by the anon_vma lock + PG_lock.
    > */
    > - down_write(&mm->mmap_sem);
    > + down_read(&mm->mmap_sem);
    > if (unlikely(khugepaged_test_exit(mm))) {
    > result = SCAN_ANY_PROCESS;
    > goto out;
    > @@ -2490,9 +2500,20 @@ static void collapse_huge_page(struct mm_struct *mm,
    > * Don't perform swapin readahead when the system is under pressure,
    > * to avoid unnecessary resource consumption.
    > */
    > - if (allocstall == curr_allocstall && swap != 0)
    > - __collapse_huge_page_swapin(mm, vma, address, pmd);
    > + if (allocstall == curr_allocstall && swap != 0) {
    > + /*
    > + * __collapse_huge_page_swapin always returns with mmap_sem
    > + * locked. If it fails, release mmap_sem and jump directly
    > + * label out. Continuing to collapse causes inconsistency.
    > + */
    > + if (!__collapse_huge_page_swapin(mm, vma, address, pmd)) {
    > + up_read(&mm->mmap_sem);
    > + goto out;
    > + }
    > + }
    >
    > + up_read(&mm->mmap_sem);
    > + down_write(&mm->mmap_sem);

    That's the critical point.

    How do you guarantee that the vma will not be destroyed (or changed)
    between up_read() and down_write()?

    You need at least find_vma() again.

    > anon_vma_lock_write(vma->anon_vma);
    >
    > pte = pte_offset_map(pmd, address);
    > --
    > 1.9.1
    >
    > --
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    > the body to majordomo@kvack.org. For more info on Linux MM,
    > see: http://www.linux-mm.org/ .
    > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

    --
    Kirill A. Shutemov

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