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Shutemov" Subject: [PATCH v4 3/9] mm: pagewalk: Don't split transhuge pmds when a pmd_entry is present Date: Tue, 8 Oct 2019 11:15:02 +0200 Message-Id: <20191008091508.2682-4-thomas_os@shipmail.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20191008091508.2682-1-thomas_os@shipmail.org> References: <20191008091508.2682-1-thomas_os@shipmail.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Thomas Hellstrom 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. 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 Cc: Will Deacon Cc: Peter Zijlstra Cc: Rik van Riel Cc: Minchan Kim Cc: Michal Hocko Cc: Huang Ying Cc: Jérôme Glisse Cc: Kirill A. Shutemov Suggested-by: Linus Torvalds Signed-off-by: Thomas Hellstrom --- 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 +/* 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) + 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. * @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