On Tue, Feb 18, 2025 at 05:27:24PM +0100, David Hildenbrand wrote: > On 18.02.25 17:21, Lorenzo Stoakes wrote: > > On Tue, Feb 18, 2025 at 05:17:20PM +0100, David Hildenbrand wrote: > > > On 18.02.25 17:12, Lorenzo Stoakes wrote: > > > > 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. > > > > > > Well, yes, (a) if there is swap and (b), if the noswap option was not > > > specified for tmpfs. > > > > > > > Right, yeah if you don't have it set up such that dropping a reference to the > > folio doesn't drop the page altogether. > > > > I think this matches expectation though in that you'd get the same results from > > an MADV_DONTNEED followed by faulting the page again. > > It might make sense to document that: installing a guard behaves just like > MADV_DONTNEED; in case of a file, that means that the pagecache is left > untouched. More docs noooo! :P I will update the man pages when this is more obviously heading for landing in 6.15 accordingly. Current man page documentation on this is: 'If the region maps memory pages those mappings will be replaced as part of the operation' I think something like: 'If the region maps pages those mappings will be replaced as part of the operation. When guard regions are removed via MADV_GUARD_REMOVE, faulting in the page will behave as if that region had MADV_DONTNEED applied to it, that is anonymous ranges will be backed by newly allocated zeroed pages and file-backed ranges will be backed by the underlying file pages.' Probably something less wordy than this... > > > > > > Okay, so installing a guard entry might require punshing a hole to get rid > > > of any already-existing memory. But readahead (below) might mess it up. > > > > Only if you are so concerned about avoiding the page cache being populated there > > that you want to do this :) > > > > Readahead I think will not readahead into a holepunched region as the hole > > punching extends to the fs layer _I believe_ I have not checked the code for > > this, but I believe it actually changes the underlying file too right to say > > 'this part of the file is empty'? > > Well, we are talking about shmem here ... not your ordinary fs backed by an > actual file :) I am talking about both, I multitask ;) > > -- > Cheers, > > David / dhildenb >
