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SubjectRe: [PATCH v4 3/9] mm: pagewalk: Don't split transhuge pmds when a pmd_entry is present
On Tue, Oct 08, 2019 at 11:15:02AM +0200, Thomas Hellström (VMware) wrote:
> From: Thomas Hellstrom <thellstrom@vmware.com>
>
> The pagewalk code was unconditionally splitting transhuge pmds when a
> pte_entry was present. However ideally we'd want to handle transhuge pmds
> in the pmd_entry function and ptes in pte_entry function. So don't split
> huge pmds when there is a pmd_entry function present, but let the callback
> take care of it if necessary.

Do we have any current user that expect split_huge_pmd() in this scenario.

>
> In order to make sure a virtual address range is handled by one and only
> one callback, and since pmd entries may be unstable, we introduce a
> pmd_entry return code that tells the walk code to continue processing this
> pmd entry rather than to move on. Since caller-defined positive return
> codes (up to 2) are used by current callers, use a high value that allows a
> large range of positive caller-defined return codes for future users.
>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@surriel.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Jérôme Glisse <jglisse@redhat.com>
> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
> ---
> include/linux/pagewalk.h | 8 ++++++++
> mm/pagewalk.c | 28 +++++++++++++++++++++-------
> 2 files changed, 29 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
> index bddd9759bab9..c4a013eb445d 100644
> --- a/include/linux/pagewalk.h
> +++ b/include/linux/pagewalk.h
> @@ -4,6 +4,11 @@
>
> #include <linux/mm.h>
>
> +/* Highest positive pmd_entry caller-specific return value */
> +#define PAGE_WALK_CALLER_MAX (INT_MAX / 2)
> +/* The handler did not handle the entry. Fall back to the next level */
> +#define PAGE_WALK_FALLBACK (PAGE_WALK_CALLER_MAX + 1)
> +

That's hacky.

Maybe just use an error code for this? -EAGAIN?

> struct mm_walk;
>
> /**
> @@ -16,6 +21,9 @@ struct mm_walk;
> * this handler is required to be able to handle
> * pmd_trans_huge() pmds. They may simply choose to
> * split_huge_page() instead of handling it explicitly.
> + * If the handler did not handle the PMD, or split the
> + * PMD and wants it handled by the PTE handler, it
> + * should return PAGE_WALK_FALLBACK.

Indentation is broken. Use tabs.

> * @pte_entry: if set, called for each non-empty PTE (4th-level) entry
> * @pte_hole: if set, called for each hole at all levels
> * @hugetlb_entry: if set, called for each hugetlb entry
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 83c0b78363b4..f844c2a2aa60 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -50,10 +50,18 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> * This implies that each ->pmd_entry() handler
> * needs to know about pmd_trans_huge() pmds
> */
> - if (ops->pmd_entry)
> + if (ops->pmd_entry) {
> err = ops->pmd_entry(pmd, addr, next, walk);
> - if (err)
> - break;
> + if (!err)
> + continue;
> + else if (err <= PAGE_WALK_CALLER_MAX)
> + break;
> + WARN_ON(err != PAGE_WALK_FALLBACK);
> + err = 0;
> + if (pmd_trans_unstable(pmd))
> + goto again;
> + /* Fall through */
> + }
>
> /*
> * Check this here so we only break down trans_huge
> @@ -61,8 +69,8 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> */
> if (!ops->pte_entry)
> continue;
> -
> - split_huge_pmd(walk->vma, pmd, addr);
> + if (!ops->pmd_entry)
> + split_huge_pmd(walk->vma, pmd, addr);
> if (pmd_trans_unstable(pmd))
> goto again;
> err = walk_pte_range(pmd, addr, next, walk);
> @@ -281,11 +289,17 @@ static int __walk_page_range(unsigned long start, unsigned long end,
> *
> * - 0 : succeeded to handle the current entry, and if you don't reach the
> * end address yet, continue to walk.
> - * - >0 : succeeded to handle the current entry, and return to the caller
> - * with caller specific value.
> + * - >0, and <= PAGE_WALK_CALLER_MAX : succeeded to handle the current entry,
> + * and return to the caller with caller specific value.
> * - <0 : failed to handle the current entry, and return to the caller
> * with error code.
> *
> + * For pmd_entry(), a value <= PAGE_WALK_CALLER_MAX indicates that the entry
> + * was handled by the callback. PAGE_WALK_FALLBACK indicates that the entry
> + * could not be handled by the callback and should be re-checked. If the
> + * callback needs the entry to be handled by the next level, it should
> + * split the entry and then return PAGE_WALK_FALLBACK.
> + *
> * Before starting to walk page table, some callers want to check whether
> * they really want to walk over the current vma, typically by checking
> * its vm_flags. walk_page_test() and @ops->test_walk() are used for this
> --
> 2.21.0
>

--
Kirill A. Shutemov

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