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Wed, 22 Mar 2023 09:16:27 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id ugSKL2vHGmRwXgAAMHmgww (envelope-from ); Wed, 22 Mar 2023 09:16:27 +0000 Message-ID: <08b98b4a-c6cf-b1b7-a4f8-2ebf3c3a0c67@suse.cz> Date: Wed, 22 Mar 2023 10:16:27 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0 Subject: Re: [PATCH v2 3/4] mm/mmap/vma_merge: extend invariants, avoid invalid res, vma Content-Language: en-US To: Lorenzo Stoakes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton Cc: David Hildenbrand , Matthew Wilcox , "Liam R . Howlett" , maple-tree@lists.infradead.org, Vernon Yang References: <17b6fc3edc46c4b33aa93b9ef17a63a3a76f4b5f.1679431180.git.lstoakes@gmail.com> From: Vlastimil Babka In-Reply-To: <17b6fc3edc46c4b33aa93b9ef17a63a3a76f4b5f.1679431180.git.lstoakes@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: 6puczd499uh7ikx1hzax8rsforu31de5 X-Rspam-User: X-Rspamd-Queue-Id: 8311EC0015 X-Rspamd-Server: rspam06 X-HE-Tag: 1679476589-859445 X-HE-Meta: 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 j6roFjNw P+fncHKXOEROM0fmDBAIs/DUVwMcbBBJRjJt6JrQQWWKLxXimIoTN/qrig/Kom5bvabkpwv1QyKi+wo7kYox+V3XTk7zLMltD/PT2659JQEvlYhGEV8BOdVtHm1H0AzCBzYNtqm3WHFrq+Y6dMXUWBS3yKzfp0F1q6F+GSQjGjPFtYdYexNf3Z+vZ+KkII2hzblzNz6N9fwcPiqgSCweruZ1n4Tr2P2sAFRIaidHIFMsON+mKFsYn0WEp8jkkXK85Y+eERiYtCL7fxM3mIHTVhXxaR/CQQNEGjDQv 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 3/21/23 21:45, Lorenzo Stoakes wrote: > Previously, vma was an uninitialised variable which was only definitely > assigned as a result of the logic covering all possible input cases - for > it to have remained uninitialised, prev would have to be NULL, and next > would _have_ to be mergeable. > > We now reuse vma to assign curr and next, so to be absolutely explicit, > ensure this variable is _always_ assigned, and while we're at it remove the > redundant assignment of both res and vma (if prev is NULL then we simply > assign to NULL). > > In addition, we absolutely do rely on addr == curr->vm_start should curr > exist, so assert as much. > > Signed-off-by: Lorenzo Stoakes Reviewed-by: Vlastimil Babka Nit suggestion below. > --- > mm/mmap.c | 19 ++++++++++++++----- > 1 file changed, 14 insertions(+), 5 deletions(-) > > diff --git a/mm/mmap.c b/mm/mmap.c > index 6361baf75601..7aec49c3bc74 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -911,7 +911,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm, > { > pgoff_t pglen = (end - addr) >> PAGE_SHIFT; > pgoff_t vma_pgoff; > - struct vm_area_struct *curr, *next, *res = NULL; > + struct vm_area_struct *curr, *next, *res; > struct vm_area_struct *vma, *adjust, *remove, *remove2; > int err = -1; > bool merge_prev = false; > @@ -940,14 +940,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm, > /* Is there a VMA next to a hole (case 1 - 3) or prev (4)? */ > next = vma_lookup(mm, end); > > - /* verify some invariant that must be enforced by the caller */ > + /* > + * By default, we return prev. Cases 3, 4, 8 will instead return next > + * and cases 3, 8 will also update vma to point at next. > + */ > + res = vma = prev; Later in the function there's a line: remove = remove2 = adjust = NULL; Now it would make sense to move it up here? > + > + /* Verify some invariant that must be enforced by the caller. */ > VM_WARN_ON(prev && addr <= prev->vm_start); > - VM_WARN_ON(curr && end > curr->vm_end); > + VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end)); > VM_WARN_ON(addr >= end); > > if (prev) { > - res = prev; > - vma = prev; > vma_start = prev->vm_start; > vma_pgoff = prev->vm_pgoff; > /* Can we merge the predecessor? */ > @@ -958,6 +962,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm, > vma_prev(vmi); > } > } > + > /* Can we merge the successor? */ > if (next && mpol_equal(policy, vma_policy(next)) && > can_vma_merge_before(next, vm_flags, > @@ -998,6 +1003,10 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm, > adj_start = -(prev->vm_end - addr); > err = dup_anon_vma(next, prev); > } else { > + /* > + * Note that cases 3 and 8 are the ONLY ones where prev > + * is permitted to be (but is not necessarily) NULL. > + */ > vma = next; /* case 3 */ > vma_start = addr; > vma_end = next->vm_end;