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 DYfaZnU5 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 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 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 > > --- > > 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 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 >