Re: [PATCH v1] hugetlb: support FOLL_FORCE|FOLL_WRITE

David Hildenbrand
2024-12-04T19:01:56Z

On 04.12.24 19:26, Guillaume Morin wrote:

Patch prefix should likely be "mm/hugetlb: ..."

> FOLL_FORCE|FOLL_WRITE has never been properly supported for hugetlb
> mappings. Since 1d8d14641fd94, we explicitly reject it. However

"Since commit 1d8d14641fd9 ("mm/hugetlb: support write-faults in shared
mappings") ..."

> running software on hugetlb mappings is a useful optimization.
> Multiple tools allow to use that such as Intel iodlr or
> libhugetlbfs.

It would be better to link to the actual request where people ran into
that when using PTRACE_POKETEXT

That hugetlb is getting used is rather obvious :)

>
> Cc: Muchun Song <muchun.song@linux.dev>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Eric Hagberg <ehagberg@janestreet.com>
> Signed-off-by: Guillaume Morin <guillaume@morinfr.org>
> ---

[...]

> delayacct_wpcopy_end();
> return 0;
> @@ -5943,7 +5944,8 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
> spin_lock(vmf->ptl);
> vmf->pte = hugetlb_walk(vma, vmf->address, huge_page_size(h));
> if (likely(vmf->pte && pte_same(huge_ptep_get(mm, vmf->address, vmf->pte), pte))) {
> - pte_t newpte = make_huge_pte(vma, &new_folio->page, !unshare);
> + const bool writable = !unshare && (vma->vm_flags & VM_WRITE);
> + pte_t newpte = make_huge_pte(vma, &new_folio->page, writable);
>
> /* Break COW or unshare */
> huge_ptep_clear_flush(vma, vmf->address, vmf->pte);

After rebasing to [1] this hunk here can likely be dropped.
make_huge_pte() will perform the VM_WRITE check.

[1] https://lkml.kernel.org/r/20241204153100.1967364-1-david@redhat.com

--
Cheers,

David / dhildenb