On Wed, Jan 13, 2021 at 3:05 PM Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> wrote: > > > > > Oh, that existing logic is wrong too :( Another bug. > > > > > > I do not think there is a bug. > > > > > > > You can't skip pages in the pages[] array under the assumption they > > > > are contiguous. ie the i+=step is wrong. > > > > > > If pages[i] is part of a compound page, the other parts of this page > > > must be sequential in this array for this compound page > > > > That is true only if the PMD points to the page. If the PTE points to > > a tail page then there is no requirement that other PTEs are > > contiguous with the compount page. > > > > At this point we have no idea if the GUP logic got this compound page > > as a head page in a PMD or as a tail page from a PTE, so we can't > > assume a contiguous run of addresses. > > I see, I will fix this bug in an upstream as a separate patch in my > series, and keep the fix when my fixes are applied. > > > > > Look at split_huge_pmd() - it doesn't break up the compound page it > > just converts the PMD to a PTE array and scatters the tail pages to > > the PTE. Hi Jason, I've been thinking about this some more. Again, I am not sure this is a bug. I understand split_huge_pmd() may split the PMD size page into PTEs and leave the compound page intact. However, in order for pages[] to have non sequential addresses in compound page, those PTEs must also be migrated after split_huge_pmd(), however when we migrate them we will either migrate the whole compound page or do split_huge_page_to_list() which will in turn do ClearPageCompound(). Please let me know if I am missing something. Thank you, Pasha > > Got it, unfortunately the fix will deoptimize the code by having to > check every page if it is part of a previous compound page or not. > > > > > I understand Matt is pushing on this idea more by having compound > > pages in the page cache, but still mapping tail pages when required. > > > > > This is actually standard migration procedure, elsewhere in the kernel > > > we migrate pages in exactly the same fashion: isolate and later > > > migrate. The isolation works for LRU only pages. > > > > But do other places cause a userspace visible random failure when LRU > > isolation fails? > > Makes sense, I will remove maximum retries for isolation, and retry > indefinitely, the same as it is done during memory hot-remove. So, we > will fail only when migration fails. > > > > > I don't like it at all, what is the user supposed to do? > > > > Jason
