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charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Stat-Signature: mxopi1ay7fg8xn97uqgspws9h85c8wjr X-Rspamd-Queue-Id: E78D3120003 X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1745597903-642931 X-HE-Meta: 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 7NdgarRK njD71B42qqoYQ697IMkCzhg1m4/0PPMuLtv/dPaCbnHQQYtO7kfkv0icXPQtZOMmW7j3Bw89BCwWw2iNaNQEgR+sleg== 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 Fri, Apr 25, 2025 at 12:07:31PM -0400, James Houghton wrote: > On Fri, Apr 25, 2025 at 11:58 AM David Hildenbrand wrote: > > > > On 25.04.25 17:45, James Houghton wrote: > > > On Thu, Apr 24, 2025 at 5:57 PM Peter Xu wrote: > > >> > > >> When discussing some userfaultfd issues with Andrea, Andrea pointed out an > > >> ABI issue with userfaultfd that existed for years. Luckily the issue > > >> should only be a very corner case one, and the fix (even if changing the > > >> kernel ABI) should only be in the good way, IOW there should have no risk > > >> breaking any userapp but only fixing. > > > > > > FWIW, my userspace basically looks like this: > > > > > > struct uffdio_continue uffdio_continue; > > > int64_t target_len = /* whatever */; > > > int64_t bytes_mapped = 0; > > > int ioctl_ret; > > > do { > > > uffdio_continue.range = /* whatever */; > > > uffdio_continue.mapped = 0; > > > ioctl_ret = ioctl(uffd, UFFDIO_CONTINUE, &uffdio_continue); > > > if (uffdio_continue.mapped < 0) { break; } > > > bytes_mapped += uffdio_continue.mapped; > > > } while (bytes_mapped < target_len && errno == EAGAIN); > > > > > > I think your patch would indeed break this. (Perhaps I shouldn't be > > > reading from `mapped` without first checking that errno == EAGAIN.) > > > > > > Well, that's what I would say, except in practice I never actually hit > > > the mmap_changing case while invoking UFFDIO_CONTINUE. :) > > > > Hm, but what if mfill_atomic_continue() would already return -EAGAIN > > when checking mmap_changing etc? > > > > Wouldn't code already run into an issue there? > > Ah, thanks David. You're right, my code is already broken! :( > > So given that we already have a case where -EAGAIN is put in the > output field, I change my mind, let's keep putting -EAGAIN in the > output field, and I'll go fix my code. Thanks both for the comments. AFAIU it shouldn't affect any app that doesn't use UFFD_FEATURE_EVENT_* as mentioned in cover letter. But I tend to agree a fix is good, that any app should better check ioctl retval and errno, before anything else.. -- Peter Xu