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Message-ID: <d373689b-a3f2-4c45-b291-85c58289f044@linux.dev>
Date: Thu, 5 Sep 2024 20:07:00 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
david@...hat.com, hughd@...gle.com, willy@...radead.org, vbabka@...nel.org,
akpm@...ux-foundation.org, rppt@...nel.org, vishal.moola@...il.com,
peterx@...hat.com, ryan.roberts@....com, christophe.leroy2@...soprasteria.com
Subject: Re: [PATCH v3 10/14] mm: page_vma_mapped_walk: map_pte() use
pte_offset_map_rw_nolock()
On 2024/9/4 16:40, Qi Zheng wrote:
> In the caller of map_pte(), we may modify the pvmw->pte after acquiring
> the pvmw->ptl, so convert it to using pte_offset_map_rw_nolock(). At
> this time, the pte_same() check is not performed after the pvmw->ptl held,
> so we should get pmdval and do pmd_same() check to ensure the stability of
> pvmw->pmd.
>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
> mm/page_vma_mapped.c | 24 ++++++++++++++++++++----
> 1 file changed, 20 insertions(+), 4 deletions(-)
>
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index ae5cc42aa2087..f1d73fd448708 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -13,9 +13,11 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
> return false;
> }
>
> -static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
> +static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
> + spinlock_t **ptlp)
> {
> pte_t ptent;
> + pmd_t pmdval;
>
> if (pvmw->flags & PVMW_SYNC) {
> /* Use the stricter lookup */
> @@ -25,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
> return !!pvmw->pte;
> }
>
> +again:
> /*
> * It is important to return the ptl corresponding to pte,
> * in case *pvmw->pmd changes underneath us; so we need to
> @@ -32,10 +35,11 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
> * proceeds to loop over next ptes, and finds a match later.
> * Though, in most cases, page lock already protects this.
> */
> - pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
> - pvmw->address, ptlp);
> + pvmw->pte = pte_offset_map_rw_nolock(pvmw->vma->vm_mm, pvmw->pmd,
> + pvmw->address, &pmdval, ptlp);
> if (!pvmw->pte)
> return false;
> + *pmdvalp = pmdval;
>
> ptent = ptep_get(pvmw->pte);
>
> @@ -69,6 +73,12 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
> }
> pvmw->ptl = *ptlp;
> spin_lock(pvmw->ptl);
> +
> + if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pvmw->pmd)))) {
> + spin_unlock(pvmw->ptl);
Forgot to clear pvmw->ptl? Or how about moving the assignment for it
to the place where the pmd_same check is successful?
> + goto again;
> + }
> +
Maybe here is the right place to assign pvmw->ptl.
Muchun,
Thanks.
> return true;
> }
>
> @@ -278,7 +288,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> step_forward(pvmw, PMD_SIZE);
> continue;
> }
> - if (!map_pte(pvmw, &ptl)) {
> + if (!map_pte(pvmw, &pmde, &ptl)) {
> if (!pvmw->pte)
> goto restart;
> goto next_pte;
> @@ -307,6 +317,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> if (!pvmw->ptl) {
> pvmw->ptl = ptl;
> spin_lock(pvmw->ptl);
> + if (unlikely(!pmd_same(pmde, pmdp_get_lockless(pvmw->pmd)))) {
> + pte_unmap_unlock(pvmw->pte, pvmw->ptl);
> + pvmw->ptl = NULL;
> + pvmw->pte = NULL;
> + goto restart;
> + }
> }
> goto this_pte;
> } while (pvmw->address < end);
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