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Message-ID: <8395af7d-328a-425c-84a7-517e78a602b1@redhat.com>
Date: Fri, 6 Dec 2024 17:03:55 +0100
From: David Hildenbrand <david@...hat.com>
To: Guillaume Morin <guillaume@...infr.org>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Muchun Song <muchun.song@...ux.dev>,
Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
Eric Hagberg <ehagberg@...estreet.com>
Subject: Re: [PATCH v4] mm/hugetlb: support FOLL_FORCE|FOLL_WRITE
On 06.12.24 15:49, Guillaume Morin wrote:
> Eric reported that PTRACE_POKETEXT fails when applications use hugetlb
> for mapping text using huge pages. Before commit 1d8d14641fd9
> ("mm/hugetlb: support write-faults in shared mappings"), PTRACE_POKETEXT
> worked by accident, but it was buggy and silently ended up mapping pages
> writable into the page tables even though VM_WRITE was not set.
>
> In general, FOLL_FORCE|FOLL_WRITE does currently not work with hugetlb.
> Let's implement FOLL_FORCE|FOLL_WRITE properly for hugetlb, such that
> what used to work in the past by accident now properly works, allowing
> applications using hugetlb for text etc. to get properly debugged.
>
> This change might also be required to implement uprobes support for
> hugetlb [1].
>
> [1] https://lore.kernel.org/lkml/ZiK50qob9yl5e0Xz@bender.morinfr.org/
>
> Cc: Muchun Song <muchun.song@...ux.dev>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Eric Hagberg <ehagberg@...estreet.com>
> Signed-off-by: Guillaume Morin <guillaume@...infr.org>
> ---
> Changes in v2:
> - Improved commit message
> Changes in v3:
> - Fix potential unitialized mem access in follow_huge_pud
> - define pud_soft_dirty when soft dirty is not enabled
> Changes in v4:
> - Remove the soft dirty pud check
> - Remove the pud_soft_dirty added in v3
>
> mm/gup.c | 95 +++++++++++++++++++++++++---------------------------
> mm/hugetlb.c | 20 ++++++-----
> 2 files changed, 57 insertions(+), 58 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 746070a1d8bf..63c705ff4162 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -587,6 +587,33 @@ static struct folio *try_grab_folio_fast(struct page *page, int refs,
> }
> #endif /* CONFIG_HAVE_GUP_FAST */
>
> +/* Common code for can_follow_write_* */
> +static inline bool can_follow_write_common(struct page *page,
> + struct vm_area_struct *vma, unsigned int flags)
> +{
> + /* Maybe FOLL_FORCE is set to override it? */
> + if (!(flags & FOLL_FORCE))
> + return false;
> +
> + /* But FOLL_FORCE has no effect on shared mappings */
> + if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
> + return false;
> +
> + /* ... or read-only private ones */
> + if (!(vma->vm_flags & VM_MAYWRITE))
> + return false;
> +
> + /* ... or already writable ones that just need to take a write fault */
> + if (vma->vm_flags & VM_WRITE)
> + return false;
> +
> + /*
> + * See can_change_pte_writable(): we broke COW and could map the page
> + * writable if we have an exclusive anonymous page ...
> + */
> + return page && PageAnon(page) && PageAnonExclusive(page);
> +}
> +
> static struct page *no_page_table(struct vm_area_struct *vma,
> unsigned int flags, unsigned long address)
> {
> @@ -613,6 +640,18 @@ static struct page *no_page_table(struct vm_area_struct *vma,
> }
>
> #ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
> +/* FOLL_FORCE can write to even unwritable PUDs in COW mappings. */
> +static inline bool can_follow_write_pud(pud_t pud, struct page *page,
> + struct vm_area_struct *vma,
> + unsigned int flags)
> +{
> + /* If the pud is writable, we can write to the page. */
> + if (pud_write(pud))
> + return true;
> +
> + return can_follow_write_common(page, vma, flags);
> +}
> +
> static struct page *follow_huge_pud(struct vm_area_struct *vma,
> unsigned long addr, pud_t *pudp,
> int flags, struct follow_page_context *ctx)
> @@ -625,13 +664,16 @@ static struct page *follow_huge_pud(struct vm_area_struct *vma,
>
> assert_spin_locked(pud_lockptr(mm, pudp));
>
> - if ((flags & FOLL_WRITE) && !pud_write(pud))
> + pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
> + page = pfn_to_page(pfn);
> +
> + if ((flags & FOLL_WRITE) &&
> + !can_follow_write_pud(pud, page, vma, flags))
> return NULL;
>
> if (!pud_present(pud))
> return NULL;
>
> - pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
That looks wrong. See follow_huge_pmd() for reference
(1) You must not do a pfn_to_page() before we verified that we have a
present PUD.
(2) can_follow_write_pud() must be called with the first mapped page. It
would currently with hugetlb not be strictly required, but is not
future proof.
It must be likely be something like:
if (!pud_present(pud))
return NULL;
if ((flags & FOLL_WRITE) &&
!can_follow_write_pud(pud, pfn_to_page(pfn), vma, flags))
return NULL;
pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
page = pfn_to_page(pfn);
>
> 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);
);
You probably missed my earlier comment. After the recent changes to
make_huge_pte() that are already in mm/mm-unstable, this hunk can be
dropped and left unchanged. make_huge_pte() will perform the VM_WRITE check.
--
Cheers,
David / dhildenb
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