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a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=kj9zAlcOel0A:10 a=vUbySO9Y5rIA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=yPCof4ZbAAAA:8 a=fwCTt19ZY0avrvX1JkcA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-GUID: tCUJ40QyN54XzFFCH7TOzvYTdvVOPRU2 X-Proofpoint-ORIG-GUID: tCUJ40QyN54XzFFCH7TOzvYTdvVOPRU2 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTE0MDA4MiBTYWx0ZWRfX2XNQEqt3zEqQ AokWLugVwaNqaQ45hsmFxSh0JGruHJ8loBPPZbkvRgi4VTsKvOMcjF0BfKNpTmdiN9qsRufHeYP Uu0b2KgCJZ2f5ULfIJNzQGo/PpqNqypuRDYCw7nGoOIW9XrjOfMAnmwGZq3n3wUJ1ztzjsYOa32 VM1Fr46vteeM2rzOFjYVpCn4kRIN6qtO9CKDjHokCVb1sxda773ZA/paDfbrovvtKT+4c39U8gf lCJVbKj3OE+79En2vSzUhjXfVLfuJutJp8B9Kk/75Uuv78YG4BPgmAlBXs0USzExi9DNNCUfiNy PUhtn06i5H+0iRGgcTDwTmhKPbTCaBQY1luuhRKDUrMdrClyHRZSSr9JbsC0DmGZYjhRIP+oYtv LkN0PLRxMF4ekvBgpugREJHEUstFPf7FIPXo3TZmQDZThF8LQFkpqYwKFGcO4usOvesx3l2rJ4+ zFMe0jox9MixVUwQTAA== On Tue, Jan 13, 2026 at 02:08:51PM -0800, Chris Mason wrote: > On Mon, 10 Nov 2025 17:22:58 +0000 Lorenzo Stoakes wrote: > > > We only need to keep the page table stable so we can perform this operation > > under the VMA lock. PTE installation is stabilised via the PTE lock. > > > > One caveat is that, if we prepare vma->anon_vma we must hold the mmap read > > lock. We can account for this by adapting the VMA locking logic to > > explicitly check for this case and prevent a VMA lock from being acquired > > should it be the case. > > > > This check is safe, as while we might be raced on anon_vma installation, > > this would simply make the check conservative, there's no way for us to see > > an anon_vma and then for it to be cleared, as doing so requires the > > mmap/VMA write lock. > > > > We abstract the VMA lock validity logic to is_vma_lock_sufficient() for > > this purpose, and add prepares_anon_vma() to abstract the anon_vma logic. > > > > In order to do this we need to have a way of installing page tables > > explicitly for an identified VMA, so we export walk_page_range_vma() in an > > unsafe variant - walk_page_range_vma_unsafe() and use this should the VMA > > read lock be taken. > > > > We additionally update the comments in madvise_guard_install() to more > > accurately reflect the cases in which the logic may be reattempted, > > specifically THP huge pages being present. > > > > Hi Lorenzo, one more flagged by AI: > > > diff --git a/mm/madvise.c b/mm/madvise.c > > index 7ed5bedb8f8ef..2a165e9beb5bb 100644 > > [ ... ] > > > @@ -1173,8 +1180,13 @@ static long madvise_guard_install(struct madvise_behavior *madv_behavior) > > unsigned long nr_pages = 0; > > > > /* Returns < 0 on error, == 0 if success, > 0 if zap needed. */ > > - err = walk_page_range_mm_unsafe(vma->vm_mm, range->start, > > - range->end, &guard_install_walk_ops, &nr_pages); > > + if (madv_behavior->lock_mode == MADVISE_VMA_READ_LOCK) > > + err = walk_page_range_vma_unsafe(madv_behavior->vma, > > + range->start, range->end, &walk_ops, > > + &nr_pages); > > + else > > + err = walk_page_range_mm_unsafe(vma->vm_mm, range->start, > > + range->end, &walk_ops, &nr_pages); > > if (err < 0) > > return err; > > > > @@ -1195,8 +1207,7 @@ static long madvise_guard_install(struct madvise_behavior *madv_behavior) > > } > > When err > 0 (zap needed), zap_page_range_single() is called below > regardless of lock_mode. If lock_mode == MADVISE_VMA_READ_LOCK (only VMA > lock is held), and the zap range contains a THP PUD mapping that requires > splitting, the call chain: > > madvise_guard_install() > -> zap_page_range_single() > -> zap_page_range_single_batched() > -> unmap_single_vma() > -> unmap_page_range() > -> zap_p4d_range() > -> zap_pud_range() > > will hit mmap_assert_locked() in zap_pud_range() (mm/memory.c) when > pud_trans_huge() is true and partial PUD splitting is required. Yeah I was already aware of this, and I thought it was handled via is_valid_guard_vma(). You only get huge PUD THP for DAX or VFIO. VFIO is covered off as that's VM_SPECIAL but yeah, maybe not dev-dax... It's a stupid situation really, I don't think we should be asserting that there but I don't necessarily want to unravel some locking horror show at this point, will put on TODO to come back to that. > > The assertion will trigger a WARN_ON because only the VMA lock is held, > not the mmap lock. > > Is there an additional check needed before calling zap_page_range_single() > when in VMA lock mode, or should this path fall back to mmap lock when THP > is possible? is_valid_guard_vma() is the appropriate place. I'll add a !vma_is_dax() check there for now and send a patch out. > > > @@ -1715,9 +1763,9 @@ static enum madvise_lock_mode get_lock_mode(struct madvise_behavior *madv_behavi > > case MADV_POPULATE_READ: > > case MADV_POPULATE_WRITE: > > case MADV_COLLAPSE: > > + return MADVISE_MMAP_READ_LOCK; > > case MADV_GUARD_INSTALL: > > case MADV_GUARD_REMOVE: > > - return MADVISE_MMAP_READ_LOCK; > > case MADV_DONTNEED: > > case MADV_DONTNEED_LOCKED: > > case MADV_FREE: > > This change moves MADV_GUARD_INSTALL to use MADVISE_VMA_READ_LOCK, but > zap_page_range_single() called later in madvise_guard_install() may > require the mmap lock for THP PUD splitting as noted above. Clever little machine ;) > > Cheers, Lorenzo