| Message ID | 8fa3fb6e-2e39-cbea-c529-ee9e64c7d2d0@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: allow pte_offset_map[_lock]() to fail | expand |
On 2023/5/22 12:58, Hugh Dickins wrote: > map_pte() use pte_offset_map_nolock(), to make sure of the ptl belonging > to pte, even if pmd entry is then changed racily: page_vma_mapped_walk() > use that instead of getting pte_lockptr() later, or restart if map_pte() > found no page table. > > Signed-off-by: Hugh Dickins <hughd@google.com> > --- > mm/page_vma_mapped.c | 28 ++++++++++++++++++++++------ > 1 file changed, 22 insertions(+), 6 deletions(-) > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > index 947dc7491815..2af734274073 100644 > --- a/mm/page_vma_mapped.c > +++ b/mm/page_vma_mapped.c > @@ -13,16 +13,28 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw) > return false; > } > > -static bool map_pte(struct page_vma_mapped_walk *pvmw) > +static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp) > { > if (pvmw->flags & PVMW_SYNC) { > /* Use the stricter lookup */ > pvmw->pte = pte_offset_map_lock(pvmw->vma->vm_mm, pvmw->pmd, > pvmw->address, &pvmw->ptl); > - return true; > + *ptlp = pvmw->ptl; > + return !!pvmw->pte; > } > > - pvmw->pte = pte_offset_map(pvmw->pmd, pvmw->address); > + /* > + * It is important to return the ptl corresponding to pte, > + * in case *pvmw->pmd changes underneath us; so we need to > + * return it even when choosing not to lock, in case caller > + * 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); > + if (!pvmw->pte) > + return false; > + > if (pvmw->flags & PVMW_MIGRATION) { > if (!is_swap_pte(*pvmw->pte)) > return false; > @@ -51,7 +63,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw) > } else if (!pte_present(*pvmw->pte)) { > return false; > } > - pvmw->ptl = pte_lockptr(pvmw->vma->vm_mm, pvmw->pmd); > + pvmw->ptl = *ptlp; > spin_lock(pvmw->ptl); > return true; > } > @@ -156,6 +168,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > struct vm_area_struct *vma = pvmw->vma; > struct mm_struct *mm = vma->vm_mm; > unsigned long end; > + spinlock_t *ptl; > pgd_t *pgd; > p4d_t *p4d; > pud_t *pud; > @@ -257,8 +270,11 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > step_forward(pvmw, PMD_SIZE); > continue; > } > - if (!map_pte(pvmw)) > + if (!map_pte(pvmw, &ptl)) { > + if (!pvmw->pte) > + goto restart; Could pvmw->pmd be changed? Otherwise, how about just jumping to the retry label below? @@ -205,6 +205,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) } pvmw->pmd = pmd_offset(pud, pvmw->address); + +retry: /* * Make sure the pmd value isn't cached in a register by the * compiler and used as a stale value after we've observed a > goto next_pte; > + } > this_pte: > if (check_pte(pvmw)) > return true; > @@ -281,7 +297,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > } while (pte_none(*pvmw->pte)); > > if (!pvmw->ptl) { > - pvmw->ptl = pte_lockptr(mm, pvmw->pmd); > + pvmw->ptl = ptl; > spin_lock(pvmw->ptl); > } > goto this_pte;
On Mon, 22 May 2023, Qi Zheng wrote: > On 2023/5/22 12:58, Hugh Dickins wrote: > > map_pte() use pte_offset_map_nolock(), to make sure of the ptl belonging > > to pte, even if pmd entry is then changed racily: page_vma_mapped_walk() > > use that instead of getting pte_lockptr() later, or restart if map_pte() > > found no page table. > > > > Signed-off-by: Hugh Dickins <hughd@google.com> > > --- > > mm/page_vma_mapped.c | 28 ++++++++++++++++++++++------ > > 1 file changed, 22 insertions(+), 6 deletions(-) > > > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > > index 947dc7491815..2af734274073 100644 > > --- a/mm/page_vma_mapped.c > > +++ b/mm/page_vma_mapped.c > > @@ -156,6 +168,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk > > *pvmw) > > struct vm_area_struct *vma = pvmw->vma; > > struct mm_struct *mm = vma->vm_mm; > > unsigned long end; > > + spinlock_t *ptl; > > pgd_t *pgd; > > p4d_t *p4d; > > pud_t *pud; > > @@ -257,8 +270,11 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk > > *pvmw) > > step_forward(pvmw, PMD_SIZE); > > continue; > > } > > - if (!map_pte(pvmw)) > > + if (!map_pte(pvmw, &ptl)) { > > + if (!pvmw->pte) > > + goto restart; > > Could pvmw->pmd be changed? Otherwise, how about just jumping to the > retry label below? > > @@ -205,6 +205,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk > *pvmw) > } > > pvmw->pmd = pmd_offset(pud, pvmw->address); > + > +retry: > /* > * Make sure the pmd value isn't cached in a register by the > * compiler and used as a stale value after we've observed a You're right, that could be done, and that's where I'd have inserted the label if there were none already. I just thought the fewer goto labels the better, so reused the restart already there. If you feel strongly that it's actively misleading, I can certainly make that change; but it's too rare an occurrence to be worth optimizing for. Hugh
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c index 947dc7491815..2af734274073 100644 --- a/mm/page_vma_mapped.c +++ b/mm/page_vma_mapped.c @@ -13,16 +13,28 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw) return false; } -static bool map_pte(struct page_vma_mapped_walk *pvmw) +static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp) { if (pvmw->flags & PVMW_SYNC) { /* Use the stricter lookup */ pvmw->pte = pte_offset_map_lock(pvmw->vma->vm_mm, pvmw->pmd, pvmw->address, &pvmw->ptl); - return true; + *ptlp = pvmw->ptl; + return !!pvmw->pte; } - pvmw->pte = pte_offset_map(pvmw->pmd, pvmw->address); + /* + * It is important to return the ptl corresponding to pte, + * in case *pvmw->pmd changes underneath us; so we need to + * return it even when choosing not to lock, in case caller + * 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); + if (!pvmw->pte) + return false; + if (pvmw->flags & PVMW_MIGRATION) { if (!is_swap_pte(*pvmw->pte)) return false; @@ -51,7 +63,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw) } else if (!pte_present(*pvmw->pte)) { return false; } - pvmw->ptl = pte_lockptr(pvmw->vma->vm_mm, pvmw->pmd); + pvmw->ptl = *ptlp; spin_lock(pvmw->ptl); return true; } @@ -156,6 +168,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) struct vm_area_struct *vma = pvmw->vma; struct mm_struct *mm = vma->vm_mm; unsigned long end; + spinlock_t *ptl; pgd_t *pgd; p4d_t *p4d; pud_t *pud; @@ -257,8 +270,11 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) step_forward(pvmw, PMD_SIZE); continue; } - if (!map_pte(pvmw)) + if (!map_pte(pvmw, &ptl)) { + if (!pvmw->pte) + goto restart; goto next_pte; + } this_pte: if (check_pte(pvmw)) return true; @@ -281,7 +297,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) } while (pte_none(*pvmw->pte)); if (!pvmw->ptl) { - pvmw->ptl = pte_lockptr(mm, pvmw->pmd); + pvmw->ptl = ptl; spin_lock(pvmw->ptl); } goto this_pte;
map_pte() use pte_offset_map_nolock(), to make sure of the ptl belonging to pte, even if pmd entry is then changed racily: page_vma_mapped_walk() use that instead of getting pte_lockptr() later, or restart if map_pte() found no page table. Signed-off-by: Hugh Dickins <hughd@google.com> --- mm/page_vma_mapped.c | 28 ++++++++++++++++++++++------ 1 file changed, 22 insertions(+), 6 deletions(-)