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bh=8IzJmKEjAIaba/hEwIZsyT4gWhHmfAKxVA40dI8Q6GQ=; b=wMtnUPljm3e0Ki0kzDSgjw3Z6zNAQJPnlEwEHXw4g1/HmPI/z33eXALbFDgNAWw1nInHVd Df6XoK3lJaaaHnrYnGlfe/Peb9Iqnhk9kYqhwxfjZa/RV90DoeDhwVzdnOuFeLvBtg8sx9 /9xt4byLsPocfgTM+ke75iQ9yLJ8jqY= Authentication-Results: imf17.hostedemail.com; dkim=none ("invalid DKIM record") header.d=intel.com header.s=Intel header.b=ToPocWPe; dmarc=pass (policy=none) header.from=intel.com; arc=reject ("signature check failed: fail, {[1] = sig:microsoft.com:reject}"); spf=pass (imf17.hostedemail.com: domain of dan.j.williams@intel.com designates 134.134.136.20 as permitted sender) smtp.mailfrom=dan.j.williams@intel.com X-Rspam-User: X-Rspamd-Queue-Id: 1017B40086 X-Rspamd-Server: rspam11 X-Stat-Signature: q4zankftjaxcoa6cn83hygczyf9wt3wz X-HE-Tag: 1662745945-921189 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: Jason Gunthorpe wrote: > On Thu, Sep 08, 2022 at 12:27:06PM -0700, Dan Williams wrote: > > flag lets the fsdax core track when it has already dropped a page > > reference, but still has use for things like memory-failure to > > opportunistically use page->mapping on a 0-reference page. > > This is not straightforward, as discussed before the page->mapping is > allowed to change while the refcount is zero, so there is no generic > way to safely obtain a pointer to the address space from a 0 reference > page. Agree. > > You'd have to pass the 0 reference page into a new pgmap operation > which could obtain an appropriate internal lock to read page->mapping. Correct, that's what the memory-failure code does via dax_lock_page(). It pins the pgmap, freezes page->mapping associations via rcu_read_lock(), speculatively reads page->mapping, takes the Xarray lock, revalidates page->mapping is the one we read speculatively, and then finally locks the entry in place until the memory-failure handling completes.