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b=MIp12bdWHSMU1GrCwa+hfnX1qpV2W3u66ysoCVszd5rb+R79OlF5hj7b+DQN2bazx eh1MiE9BwStxsnUVkLn6Qwj+C+HT9h+oDHRs+YbMfBdIH7L9CU8kBn95iYGllk36Ac cfEtMHoLdsrMHgqb734Gi8d2inYB+SVJC68ZZm1A= Date: Thu, 3 Sep 2026 10:21:30 -0700 From: Andrew Morton To: Dave Hansen Cc: Kiryl Shutsemau , Vernon Yang , rick.p.edgecombe@intel.com, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, david@kernel.org, hpa@zytor.com, rmclure@linux.ibm.com, andrew+kernel@donnellan.id.au, pasha.tatashin@soleen.com, tj@kernel.org, rppt@kernel.org, yu-cheng.yu@intel.com, orsonpeters@gmail.com, linux-kernel@vger.kernel.org, x86@kernel.org, linux-mm@kvack.org, Vernon Yang , stable@vger.kernel.org Subject: Re: [PATCH] x86/mm: Fix pmd_modify() dropping the dirty bit Message-Id: <20260903102130.b5cda1b08f3efd4ab66226e9@linux-foundation.org> In-Reply-To: <314f11c1-ce5e-4b36-b296-0ab742550851@intel.com> References: <20260903031608.1194238-1-vernon2gm@gmail.com> <314f11c1-ce5e-4b36-b296-0ab742550851@intel.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Stat-Signature: tmaj19e9xjwfisi9wiccb7phzbkf4prj X-Rspamd-Queue-Id: 4BFA52000B X-Rspamd-Server: rspam02 X-Rspam-User: X-HE-Tag: 1788456093-179905 X-HE-Meta: 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 RrFFku1l E9dmdVytdrWrNvAdKkIJPj7h8QKfeDtjsycs9YYQriuHTpfNF1YFdLtASkJEc8axl0mwq3LqvtrUem8JpwfXRWvGz65O1OUm5m1/vJg3nb5idLNJwD58LGslbGgwGqiUAykuGNjKiaEtutIn307jF+HyyJAfmVwgPlnfI/iT6hKE0U0hkFC1bNNwvfnPmuTETznlA1siNHJPTSRw3fVYWBkm8G+mpPBtBP/D1OG0vQcbUOwei7MRdRB+vLsMTxyGoOtdwxhTXXTPs19xv+NPXp1Tc0kLXVyV4GWdIix7iTohSaz4xxVqcpQJqPnVBfoXSfU3YpvMVsqv0s2Ay0LpONuAjeQjROSjHeciFHdCgTGPmHGBt3dqg5QO+B7lUPvK44dQo4GZcM6+G/Z1GB60+bwX0PXdi6tzu218Cz1e/C5ufQNq8H/GK4dt1BIW1Kp6lu6LYPoGu8s2OoXc6bPxXaZhr2wYdb6Xb3fhLZzYHqId/MZs= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, 3 Sep 2026 07:18:54 -0700 Dave Hansen wrote: > On 9/3/26 04:54, Kiryl Shutsemau wrote: > >> Closes: https://lore.kernel.org/r/CAJxLxMUGu1-L+O_nAONOwOXnS=cNbNApCWqdthRjd76LThtSPg@mail.gmail.com/ > >> Fixes: bb3aadf7d446 ("x86/mm: Start actually marking _PAGE_SAVED_DIRTY") > > Hm. I don't understand why would this commit explicitly exclude > > _PAGE_DIRTY from the mask: > > > > - val &= _HPAGE_CHG_MASK; > > + val &= (_HPAGE_CHG_MASK & ~_PAGE_DIRTY); > > > > Rick, could you comment? It doesn't look like a typo. > > My guess is that it's some remnant from an earlier version of the patch. > The asymmetry with pte_modify() vs. pmd_modify() just can't be explained > any other way. It _might_ have been some attempt to say, "Hey > _PAGE_DIRTY is now a part of the pgprot_t since it's part of the > 'permissions' of a shadow stack PTE" that got abandoned. > > But I don't see anything wrong with the fix at all. > > It does give me pause that this has been losing user data for so long, > but it must just be a weird combination of features that few folks use > together (huge pages + MADV_FREE). > And with heavy reclaim pressure.