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[195.135.220.15]) by mx.google.com with ESMTPS id l22si10219017ejb.1.2019.07.16.01.12.01 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Jul 2019 01:12:01 -0700 (PDT) Received-SPF: pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id A333CABED; Tue, 16 Jul 2019 08:12:00 +0000 (UTC) Subject: Re: [v2 PATCH 1/2] mm: mempolicy: make the behavior consistent when MPOL_MF_MOVE* and MPOL_MF_STRICT were specified To: Yang Shi , mhocko@kernel.org, mgorman@techsingularity.net, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1561162809-59140-1-git-send-email-yang.shi@linux.alibaba.com> <1561162809-59140-2-git-send-email-yang.shi@linux.alibaba.com> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <1561162809-59140-2-git-send-email-yang.shi@linux.alibaba.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 6/22/19 2:20 AM, Yang Shi wrote: > When both MPOL_MF_MOVE* and MPOL_MF_STRICT was specified, mbind() should > try best to migrate misplaced pages, if some of the pages could not be > migrated, then return -EIO. > > There are three different sub-cases: > 1. vma is not migratable > 2. vma is migratable, but there are unmovable pages > 3. vma is migratable, pages are movable, but migrate_pages() fails > > If #1 happens, kernel would just abort immediately, then return -EIO, > after the commit a7f40cfe3b7ada57af9b62fd28430eeb4a7cfcb7 ("mm: > mempolicy: make mbind() return -EIO when MPOL_MF_STRICT is specified"). > > If #3 happens, kernel would set policy and migrate pages with best-effort, > but won't rollback the migrated pages and reset the policy back. > > Before that commit, they behaves in the same way. It'd better to keep > their behavior consistent. But, rolling back the migrated pages and > resetting the policy back sounds not feasible, so just make #1 behave as > same as #3. > > Userspace will know that not everything was successfully migrated (via > -EIO), and can take whatever steps it deems necessary - attempt rollback, > determine which exact page(s) are violating the policy, etc. > > Make queue_pages_range() return 1 to indicate there are unmovable pages > or vma is not migratable. > > The #2 is not handled correctly in the current kernel, the following > patch will fix it. > > Cc: Vlastimil Babka > Cc: Michal Hocko > Cc: Mel Gorman > Signed-off-by: Yang Shi Agreed with the goal, but I think there's a bug, and room for improvement. > --- > mm/mempolicy.c | 86 +++++++++++++++++++++++++++++++++++++++++----------------- > 1 file changed, 61 insertions(+), 25 deletions(-) > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index 01600d8..b50039c 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -429,11 +429,14 @@ static inline bool queue_pages_required(struct page *page, > } > > /* > - * queue_pages_pmd() has three possible return values: > + * queue_pages_pmd() has four possible return values: > + * 2 - there is unmovable page, and MPOL_MF_MOVE* & MPOL_MF_STRICT were > + * specified. > * 1 - pages are placed on the right node or queued successfully. > * 0 - THP was split. I think if you renumbered these, it would be more consistent with queue_pages_pte_range() and simplify the code there. 0 - pages on right node/queued 1 - unmovable page with right flags specified 2 - THP split > - * -EIO - is migration entry or MPOL_MF_STRICT was specified and an existing > - * page was already on a node that does not follow the policy. > + * -EIO - is migration entry or only MPOL_MF_STRICT was specified and an > + * existing page was already on a node that does not follow the > + * policy. > */ > static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr, > unsigned long end, struct mm_walk *walk) > @@ -463,7 +466,7 @@ static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr, > /* go to thp migration */ > if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) { > if (!vma_migratable(walk->vma)) { > - ret = -EIO; > + ret = 2; > goto unlock; > } > > @@ -488,16 +491,29 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr, Perhaps this function now also deserves a list of possible return values. > struct queue_pages *qp = walk->private; > unsigned long flags = qp->flags; > int ret; > + bool has_unmovable = false; > pte_t *pte; > spinlock_t *ptl; > > ptl = pmd_trans_huge_lock(pmd, vma); > if (ptl) { > ret = queue_pages_pmd(pmd, ptl, addr, end, walk); > - if (ret > 0) > + switch (ret) { With renumbering suggested above, this could be: if (ret != 2) return ret; > + /* THP was split, fall through to pte walk */ > + case 0: > + break; > + /* Pages are placed on the right node or queued successfully */ > + case 1: > return 0; > - else if (ret < 0) > + /* > + * Met unmovable pages, MPOL_MF_MOVE* & MPOL_MF_STRICT > + * were specified. > + */ > + case 2: > + return 1; > + case -EIO: > return ret; > + } > } > > if (pmd_trans_unstable(pmd)) > @@ -519,14 +535,21 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr, > if (!queue_pages_required(page, qp)) > continue; > if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) { > - if (!vma_migratable(vma)) > + /* MPOL_MF_STRICT must be specified if we get here */ > + if (!vma_migratable(vma)) { > + has_unmovable |= true; > break; > + } > migrate_page_add(page, qp->pagelist, flags); > } else > break; > } > pte_unmap_unlock(pte - 1, ptl); > cond_resched(); > + > + if (has_unmovable) > + return 1; > + > return addr != end ? -EIO : 0; > } > > @@ -639,7 +662,13 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end, > * > * If pages found in a given range are on a set of nodes (determined by > * @nodes and @flags,) it's isolated and queued to the pagelist which is > - * passed via @private.) > + * passed via @private. > + * > + * queue_pages_range() has three possible return values: > + * 1 - there is unmovable page, but MPOL_MF_MOVE* & MPOL_MF_STRICT were > + * specified. > + * 0 - queue pages successfully or no misplaced page. > + * -EIO - there is misplaced page and only MPOL_MF_STRICT was specified. > */ > static int > queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end, > @@ -1182,6 +1211,7 @@ static long do_mbind(unsigned long start, unsigned long len, > struct mempolicy *new; > unsigned long end; > int err; > + int ret; > LIST_HEAD(pagelist); > > if (flags & ~(unsigned long)MPOL_MF_VALID) > @@ -1243,26 +1273,32 @@ static long do_mbind(unsigned long start, unsigned long len, > if (err) > goto mpol_out; > > - err = queue_pages_range(mm, start, end, nmask, > + ret = queue_pages_range(mm, start, end, nmask, > flags | MPOL_MF_INVERT, &pagelist); > - if (!err) > - err = mbind_range(mm, start, end, new); > - > - if (!err) { > - int nr_failed = 0; > > - if (!list_empty(&pagelist)) { > - WARN_ON_ONCE(flags & MPOL_MF_LAZY); > - nr_failed = migrate_pages(&pagelist, new_page, NULL, > - start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND); > - if (nr_failed) > - putback_movable_pages(&pagelist); > - } > + if (ret < 0) > + err = -EIO; I think after your patch, you miss putback_movable_pages() in cases where some were queued, and later the walk returned -EIO. The previous code doesn't miss it, but it's also not obvious due to the multiple if (!err) checks. I would rewrite it some thing like this: if (ret < 0) { putback_movable_pages(&pagelist); err = ret; goto mmap_out; // a new label above up_write() } The rest can have reduced identation now. > + else { > + err = mbind_range(mm, start, end, new); > > - if (nr_failed && (flags & MPOL_MF_STRICT)) > - err = -EIO; > - } else > - putback_movable_pages(&pagelist); > + if (!err) { > + int nr_failed = 0; > + > + if (!list_empty(&pagelist)) { > + WARN_ON_ONCE(flags & MPOL_MF_LAZY); > + nr_failed = migrate_pages(&pagelist, new_page, > + NULL, start, MIGRATE_SYNC, > + MR_MEMPOLICY_MBIND); > + if (nr_failed) > + putback_movable_pages(&pagelist); > + } > + > + if ((ret > 0) || > + (nr_failed && (flags & MPOL_MF_STRICT))) > + err = -EIO; > + } else > + putback_movable_pages(&pagelist); While at it, IIRC the kernel style says that when the 'if' part uses '{ }' then the 'else' part should as well, and it shouldn't be mixed. Thanks, Vlastimil > + } > > up_write(&mm->mmap_sem); > mpol_out: >