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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id v14-20020a05622a014e00b002cf75f5b11esm4508896qtw.64.2022.03.04.18.29.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 04 Mar 2022 18:29:06 -0800 (PST) Date: Fri, 4 Mar 2022 18:28:39 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Liam Howlett cc: Hugh Dickins , Andrew Morton , Oleg Nesterov , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" Subject: Re: [PATCH mmotm] mempolicy: mbind_range() set_policy() after vma_merge() In-Reply-To: <20220305020021.qmwg5dkham4lyz6v@revolver> Message-ID: <29eac73-4f94-1688-3834-8bd6687a18@google.com> References: <319e4db9-64ae-4bca-92f0-ade85d342ff@google.com> <20220304184927.vkq6ewn6uqtcesma@revolver> <20220304190531.6giqbnnaka4xhovx@revolver> <6038ebc2-bc88-497d-a3f3-5936726fb023@google.com> <20220305020021.qmwg5dkham4lyz6v@revolver> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspamd-Queue-Id: D861B120004 X-Stat-Signature: bwrjtisn3os4h5a56wz6np9gmbd98hbu Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=AkMbkY1e; spf=pass (imf29.hostedemail.com: domain of hughd@google.com designates 209.85.160.181 as permitted sender) smtp.mailfrom=hughd@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1646447347-955786 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 Sat, 5 Mar 2022, Liam Howlett wrote: > * Hugh Dickins [220304 17:48]: > > On Fri, 4 Mar 2022, Liam Howlett wrote: > > > * Liam R. Howlett [220304 13:49]: > > > > * Hugh Dickins [220303 23:36]: > > > > > > I just thought of something after my initial email > > > > > > How does the ->set_policy() requirement on tmpfs play out for the > > > mpol_equal() check earlier in that for loop? > > > > It took me a while to page all this back in (and remind myself of > > what is case 8) to answer that question! > > > > The answer is that the mpol_equal() check at the top of the loop is on > > an existing, unmodified vma; so it's right to assume that any necessary > > set_policy() has already been done. > > > > Whereas the mpol_equal() check being removed in this patch, is being > > done on a vma which may have just been extended to cover a greater range: > > so although the relevant set_policy() may have already been done on a part > > of its range, there is now another part which needs the policy applied. > > Doesn't the policy get checked during vma_merge()? Specifically the > mpol_equal(policy, vma_policy(next)) check? Sorry, I'm reduced to the unhelpful reply of "Yes. So?" If vma_merge() finds that vma's new_pol allows it to be merged with prev, that still requires mbind_range() (or its call to vma_replace_policy()) to set_policy() on prev (now assigned to vma), to apply that new_pol to the extension of prev - vma_merge() would have checked mpol_equal(), but would not have done the set_policy(). Hugh