On Wed, Jan 15, 2025 at 12:21:15PM -0500, Peter Xu wrote: > On Wed, Jan 15, 2025 at 04:58:06PM +0000, Ryan Roberts wrote: > > Hi Peter, David, > > Hey, Ryan, > > > > > On 07/01/2025 14:47, Ryan Roberts wrote: > > > When mremap()ing a memory region previously registered with userfaultfd > > > as write-protected but without UFFD_FEATURE_EVENT_REMAP, an > > > inconsistency in flag clearing leads to a mismatch between the vma flags > > > (which have uffd-wp cleared) and the pte/pmd flags (which do not have > > > uffd-wp cleared). This mismatch causes a subsequent mprotect(PROT_WRITE) > > > to trigger a warning in page_table_check_pte_flags() due to setting the > > > pte to writable while uffd-wp is still set. > > > > > > Fix this by always explicitly clearing the uffd-wp pte/pmd flags on any > > > such mremap() so that the values are consistent with the existing > > > clearing of VM_UFFD_WP. Be careful to clear the logical flag regardless > > > of its physical form; a PTE bit, a swap PTE bit, or a PTE marker. Cover > > > PTE, huge PMD and hugetlb paths. > > > > I just noticed that Andrew sent this to Linus and it's now in his tree; I'm > > suddenly very nervous that it doesn't have any acks. I don't suppose you would > > be able to do a quick review to calm the nerves?? > > Heh, I fully trusted you, and I appreciated your help too. I'll need to run > for 1-2 hours, but I'll read it this afternoon. > > Side note: no review is as good as tests on reliability POV if that was the > concern, but I'll try my best. Things go all inception though when part of the review _are_ the tests ;) Though of course there are also all existing uffd tests and the bots that add a bit of weight. This isn't really my area so will defer to Peter on the review side. I sort of favour putting hotfixes in quick, but this one has gone in quicker than some reviewed hotfixes which we left in unstable... however towards the end of a cycle I think Andrew is stuck between a rock and a hard place in deciding how to handle these. So I'm guessing the heuristic is 'allow to simmer in unstable if time permits in cycle', if known 'good egg' + no objection + towards end of cycle + hotfix - send. I do wonder whether we should require review on hotfixes generally. But then of course that creates rock + hard place decision for Andrew as to whether it gets deferred to the next cycle + stable backports... Maybe one to discuss at LSF? > > Thanks, > > -- > Peter Xu >
