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b=2FgGrleEGrCNxRIzj+tCRANqs6O3JKU4VTeoxmW+CL5prrpYnZfCg3C48cXy1UjxrOdgVw vksaiPIeukDyypAlKhTgxvnidyctytM6CxQTztF9dRwK2Fx8YAP3cmiBIQU/VO4KCMJNlW JubJ6PPlJ9O+uksYBjcfhRCXashHkjM= X-Stat-Signature: y6dcddppotmhpom7651fphofzth8jg6z X-Rspamd-Queue-Id: 084F0400AC Authentication-Results: imf27.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=rec5Y+6m; spf=pass (imf27.hostedemail.com: domain of jhubbard@nvidia.com designates 40.107.223.58 as permitted sender) smtp.mailfrom=jhubbard@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com; arc=pass ("microsoft.com:s=arcselector9901:i=1") X-Rspamd-Server: rspam11 X-Rspam-User: X-HE-Tag: 1662499674-860782 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 9/6/22 11:50, Yang Shi wrote: >>> - if (unlikely(pte_val(pte) != pte_val(*ptep))) { >>> + /* >>> + * THP collapse conceptually does: >>> + * 1. Clear and flush PMD >>> + * 2. Check the base page refcount >>> + * 3. Copy data to huge page >>> + * 4. Clear PTE >>> + * 5. Discard the base page >>> + * >>> + * So fast GUP may race with THP collapse then pin and >>> + * return an old page since TLB flush is no longer sufficient >>> + * to serialize against fast GUP. >>> + * >>> + * Check PMD, if it is changed just back off since it >>> + * means there may be parallel THP collapse. >>> + */ >> >> As I mentioned in the other thread, it would be a nice touch to move >> such discussion into the comment header. > > Sure, you mean the comment before gup_pte_range() so that the real > code stays succinct, right? Yes. thanks, -- John Hubbard NVIDIA