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[35.187.0.65]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3502b79bc3bsm31525781f8f.13.2024.05.21.03.10.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 May 2024 03:10:20 -0700 (PDT) Date: Tue, 21 May 2024 11:10:16 +0100 From: Vincent Donnefort To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Dan Williams , rostedt@goodmis.org Subject: Re: [PATCH v1 1/2] mm/memory: cleanly support zeropage in vm_insert_page*(), vm_map_pages*() and vmf_insert_mixed() Message-ID: References: <20240430204044.52755-1-david@redhat.com> <20240430204044.52755-2-david@redhat.com> <3decc6c8-9035-44d6-89c6-8d42a5e0bc40@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: zg9torzjby7mnq3semhahzorqjt4ibqo X-Rspamd-Queue-Id: A55E120003 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1716286222-853379 X-HE-Meta: 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 GGWUciuN cvQ3QlvFuhCPuU6LveS6Heg6pTuylKeVuuRB6ORqXbgtwXoAatIW3X2aldT3ULHwbe+1+53DL5ma6JYU+W5xjrjl207IDUTSk3FTcqq1/AVu92xyMAbZpoHZuNjxROt3GtPwyOm3kPDc34QYm/KF4tfmKpm8LJ7etLrfF6Ozyrk8H9s6xxT2L678uY1SH+6yKO0ojrryvuHYnGTu6eCYj+tnQ2tFs0zOaDTzamM8xaFyrR58SUqNPXjhIK2TznVIwke2DRtSO+iP3mOGR7jDxI8Vw0EtRrUGkTlwJCIn1+FGeN9u946jvjQEHvj66At5kmUWLYdbYgIYCmipK8Zi1quUDhN5/loVnx4HE3tkO30uUhaMcXh6EFMMXihzBFwbNbYkGGbsRbtfkImTVB4S216/72g== 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: List-Subscribe: List-Unsubscribe: On Tue, May 21, 2024 at 11:18:41AM +0200, David Hildenbrand wrote: > On 21.05.24 11:06, Vincent Donnefort wrote: > > On Tue, May 21, 2024 at 10:25:43AM +0200, David Hildenbrand wrote: > > > On 17.05.24 17:07, Vincent Donnefort wrote: > > > > Hi David, > > > > > > > > [...] > > > > > > > > > -static int validate_page_before_insert(struct page *page) > > > > > +static bool vm_mixed_zeropage_allowed(struct vm_area_struct *vma) > > > > > +{ > > > > > + VM_WARN_ON_ONCE(vma->vm_flags & VM_PFNMAP); > > > > > + /* > > > > > + * Whoever wants to forbid the zeropage after some zeropages > > > > > + * might already have been mapped has to scan the page tables and > > > > > + * bail out on any zeropages. Zeropages in COW mappings can > > > > > + * be unshared using FAULT_FLAG_UNSHARE faults. > > > > > + */ > > > > > + if (mm_forbids_zeropage(vma->vm_mm)) > > > > > + return false; > > > > > + /* zeropages in COW mappings are common and unproblematic. */ > > > > > + if (is_cow_mapping(vma->vm_flags)) > > > > > + return true; > > > > > + /* Mappings that do not allow for writable PTEs are unproblematic. */ > > > > > + if (!(vma->vm_flags & (VM_WRITE | VM_MAYWRITE))) > > > > > + return false; > > > > > > > > Shouldn't we return true here? > > > > > > Indeed, thanks! I wish we would have user in the tree already that could > > > exercise that code path. > > > > I have a patch ready to use this path from the memory map tracing! I can either > > send it once this one is picked-up or you can add it to your series? > > Whatever works for you! To debug, it would be good if you could send me the > patch and simple instructions on how to test it (do we have a selftest as > well?). Of course, I'll share something with you today! It includes an update to the selftest to make sure we check the padding with the zero-page. > > > > > > > > > [...] > > > > > > > > @@ -2043,7 +2085,7 @@ static int insert_page_in_batch_locked(struct vm_area_struct *vma, pte_t *pte, > > > > > if (!page_count(page)) > > > > > return -EINVAL; > > > > > > > > This test here prevents inserting the zero-page. > > > > > > You mean the existing page_count() check? or the (wrong) vma->vm_flags check > > > in vm_mixed_zeropage_allowed() ? > > > > I meant this page_count() here. As a quick test, I removed that check (also fixed > > the vm_flags above) and the zero-page was properly mapped! > > That's weird and might indicate another issue. > > The refcount of the shared zeropage should be initialized to 1, just like > for any other reserved pages > (mm/mm_init.c:__init_single_page()->init_page_count()) > > Hm ... > > -- > Cheers, > > David / dhildenb >