On Tue, Feb 18, 2025 at 05:01:16PM +0100, David Hildenbrand wrote: > On 13.02.25 19:17, Lorenzo Stoakes wrote: > > There is no reason to disallow guard regions in file-backed mappings - > > readahead and fault-around both function correctly in the presence of PTE > > markers, equally other operations relating to memory-mapped files function > > correctly. > > > > Additionally, read-only mappings if introducing guard-regions, only > > restrict the mapping further, which means there is no violation of any > > access rights by permitting this to be so. > > > > Removing this restriction allows for read-only mapped files (such as > > executable files) correctly which would otherwise not be permitted. > > > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> > > --- > > mm/madvise.c | 8 +------- > > 1 file changed, 1 insertion(+), 7 deletions(-) > > > > diff --git a/mm/madvise.c b/mm/madvise.c > > index 6ecead476a80..e01e93e179a8 100644 > > --- a/mm/madvise.c > > +++ b/mm/madvise.c > > @@ -1051,13 +1051,7 @@ static bool is_valid_guard_vma(struct vm_area_struct *vma, bool allow_locked) > > if (!allow_locked) > > disallowed |= VM_LOCKED; > > - if (!vma_is_anonymous(vma)) > > - return false; > > - > > - if ((vma->vm_flags & (VM_MAYWRITE | disallowed)) != VM_MAYWRITE) > > - return false; > > - > > - return true; > > + return !(vma->vm_flags & disallowed); > > } > > static bool is_guard_pte_marker(pte_t ptent) > > Acked-by: David Hildenbrand <david@xxxxxxxxxx> Thanks! > > I assume these markers cannot completely prevent us from allocating > pages/folios for these underlying file/pageache ranges of these markers in > case of shmem during page faults, right? If the markers are in place, then page faulting will result in a segfault. If we faulted in a shmem page then installed markers (which would zap the range), then the page cache will be populated, but obviously subject to standard reclaim. If we perform synchronous readahead prior to a guard region that includes (partially or fully) a guard region we might major fault entries into the page cache that are then not accessable _from that mapping_, this is rather unavoidable as this doesn't account for page table mappings and should be largely trivial overhead (also these folios are reclaimable). > > -- > Cheers, > > David / dhildenb >
