| Message ID | 161604050866.1463742.7759521510383551055.stgit@dwillia2-desk3.amr.corp.intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm, pmem: Force unmap pmem on surprise remove | expand |
On 3/18/21 4:08 AM, Dan Williams wrote: > Now that device-dax and filesystem-dax are guaranteed to unmap all user > mappings of devmap / DAX pages before tearing down the 'struct page' > array, get_user_pages_fast() can rely on its traditional synchronization > method "validate_pte(); get_page(); revalidate_pte()" to catch races with > device shutdown. Specifically the unmap guarantee ensures that gup-fast > either succeeds in taking a page reference (lock-less), or it detects a > need to fall back to the slow path where the device presence can be > revalidated with locks held. [...] > @@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, > #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */ > > #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE) > + > static int __gup_device_huge(unsigned long pfn, unsigned long addr, > unsigned long end, unsigned int flags, > struct page **pages, int *nr) > { > int nr_start = *nr; > - struct dev_pagemap *pgmap = NULL; > > do { > - struct page *page = pfn_to_page(pfn); > + struct page *page; > + > + /* > + * Typically pfn_to_page() on a devmap pfn is not safe > + * without holding a live reference on the hosting > + * pgmap. In the gup-fast path it is safe because any > + * races will be resolved by either gup-fast taking a > + * reference or the shutdown path unmapping the pte to > + * trigger gup-fast to fall back to the slow path. > + */ > + page = pfn_to_page(pfn); > > - pgmap = get_dev_pagemap(pfn, pgmap); > - if (unlikely(!pgmap)) { > - undo_dev_pagemap(nr, nr_start, flags, pages); > - return 0; > - } > SetPageReferenced(page); > pages[*nr] = page; > if (unlikely(!try_grab_page(page, flags))) { So for allowing FOLL_LONGTERM[0] would it be OK if we used page->pgmap after try_grab_page() for checking pgmap type to see if we are in a device-dax longterm pin? Joao [0] https://lore.kernel.org/linux-mm/6a18179e-65f7-367d-89a9-d5162f10fef0@oracle.com/
On Thu, Mar 18, 2021 at 3:02 AM Joao Martins <joao.m.martins@oracle.com> wrote: > > On 3/18/21 4:08 AM, Dan Williams wrote: > > Now that device-dax and filesystem-dax are guaranteed to unmap all user > > mappings of devmap / DAX pages before tearing down the 'struct page' > > array, get_user_pages_fast() can rely on its traditional synchronization > > method "validate_pte(); get_page(); revalidate_pte()" to catch races with > > device shutdown. Specifically the unmap guarantee ensures that gup-fast > > either succeeds in taking a page reference (lock-less), or it detects a > > need to fall back to the slow path where the device presence can be > > revalidated with locks held. > > [...] > > > @@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, > > #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */ > > > > #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE) > > + > > static int __gup_device_huge(unsigned long pfn, unsigned long addr, > > unsigned long end, unsigned int flags, > > struct page **pages, int *nr) > > { > > int nr_start = *nr; > > - struct dev_pagemap *pgmap = NULL; > > > > do { > > - struct page *page = pfn_to_page(pfn); > > + struct page *page; > > + > > + /* > > + * Typically pfn_to_page() on a devmap pfn is not safe > > + * without holding a live reference on the hosting > > + * pgmap. In the gup-fast path it is safe because any > > + * races will be resolved by either gup-fast taking a > > + * reference or the shutdown path unmapping the pte to > > + * trigger gup-fast to fall back to the slow path. > > + */ > > + page = pfn_to_page(pfn); > > > > - pgmap = get_dev_pagemap(pfn, pgmap); > > - if (unlikely(!pgmap)) { > > - undo_dev_pagemap(nr, nr_start, flags, pages); > > - return 0; > > - } > > SetPageReferenced(page); > > pages[*nr] = page; > > if (unlikely(!try_grab_page(page, flags))) { > > So for allowing FOLL_LONGTERM[0] would it be OK if we used page->pgmap after > try_grab_page() for checking pgmap type to see if we are in a device-dax > longterm pin? > Yes. I still need to answer the question of whether mapping invalidation triggers longterm pin holders to relinquish their hold, but that's a problem regardless of whether gup-fast is supported or not.
On Thu, Mar 18, 2021 at 3:02 AM Joao Martins <joao.m.martins@oracle.com> wrote: > > On 3/18/21 4:08 AM, Dan Williams wrote: > > Now that device-dax and filesystem-dax are guaranteed to unmap all user > > mappings of devmap / DAX pages before tearing down the 'struct page' > > array, get_user_pages_fast() can rely on its traditional synchronization > > method "validate_pte(); get_page(); revalidate_pte()" to catch races with > > device shutdown. Specifically the unmap guarantee ensures that gup-fast > > either succeeds in taking a page reference (lock-less), or it detects a > > need to fall back to the slow path where the device presence can be > > revalidated with locks held. > > [...] > > > @@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, > > #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */ > > > > #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE) > > + > > static int __gup_device_huge(unsigned long pfn, unsigned long addr, > > unsigned long end, unsigned int flags, > > struct page **pages, int *nr) > > { > > int nr_start = *nr; > > - struct dev_pagemap *pgmap = NULL; > > > > do { > > - struct page *page = pfn_to_page(pfn); > > + struct page *page; > > + > > + /* > > + * Typically pfn_to_page() on a devmap pfn is not safe > > + * without holding a live reference on the hosting > > + * pgmap. In the gup-fast path it is safe because any > > + * races will be resolved by either gup-fast taking a > > + * reference or the shutdown path unmapping the pte to > > + * trigger gup-fast to fall back to the slow path. > > + */ > > + page = pfn_to_page(pfn); > > > > - pgmap = get_dev_pagemap(pfn, pgmap); > > - if (unlikely(!pgmap)) { > > - undo_dev_pagemap(nr, nr_start, flags, pages); > > - return 0; > > - } > > SetPageReferenced(page); > > pages[*nr] = page; > > if (unlikely(!try_grab_page(page, flags))) { > > So for allowing FOLL_LONGTERM[0] would it be OK if we used page->pgmap after > try_grab_page() for checking pgmap type to see if we are in a device-dax > longterm pin? So, there is an effort to add a new pte bit p{m,u}d_special to disable gup-fast for huge pages [1]. I'd like to investigate whether we could use devmap + special as an encoding for "no longterm" and never consult the pgmap in the gup-fast path. [1]: https://lore.kernel.org/linux-mm/a1fa7fa2-914b-366d-9902-e5b784e8428c@shipmail.org/
On 3/24/21 5:45 PM, Dan Williams wrote: > On Thu, Mar 18, 2021 at 3:02 AM Joao Martins <joao.m.martins@oracle.com> wrote: >> On 3/18/21 4:08 AM, Dan Williams wrote: >>> Now that device-dax and filesystem-dax are guaranteed to unmap all user >>> mappings of devmap / DAX pages before tearing down the 'struct page' >>> array, get_user_pages_fast() can rely on its traditional synchronization >>> method "validate_pte(); get_page(); revalidate_pte()" to catch races with >>> device shutdown. Specifically the unmap guarantee ensures that gup-fast >>> either succeeds in taking a page reference (lock-less), or it detects a >>> need to fall back to the slow path where the device presence can be >>> revalidated with locks held. >> >> [...] >> >>> @@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, >>> #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */ >>> >>> #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE) >>> + >>> static int __gup_device_huge(unsigned long pfn, unsigned long addr, >>> unsigned long end, unsigned int flags, >>> struct page **pages, int *nr) >>> { >>> int nr_start = *nr; >>> - struct dev_pagemap *pgmap = NULL; >>> >>> do { >>> - struct page *page = pfn_to_page(pfn); >>> + struct page *page; >>> + >>> + /* >>> + * Typically pfn_to_page() on a devmap pfn is not safe >>> + * without holding a live reference on the hosting >>> + * pgmap. In the gup-fast path it is safe because any >>> + * races will be resolved by either gup-fast taking a >>> + * reference or the shutdown path unmapping the pte to >>> + * trigger gup-fast to fall back to the slow path. >>> + */ >>> + page = pfn_to_page(pfn); >>> >>> - pgmap = get_dev_pagemap(pfn, pgmap); >>> - if (unlikely(!pgmap)) { >>> - undo_dev_pagemap(nr, nr_start, flags, pages); >>> - return 0; >>> - } >>> SetPageReferenced(page); >>> pages[*nr] = page; >>> if (unlikely(!try_grab_page(page, flags))) { >> >> So for allowing FOLL_LONGTERM[0] would it be OK if we used page->pgmap after >> try_grab_page() for checking pgmap type to see if we are in a device-dax >> longterm pin? > > So, there is an effort to add a new pte bit p{m,u}d_special to disable > gup-fast for huge pages [1]. I'd like to investigate whether we could > use devmap + special as an encoding for "no longterm" and never > consult the pgmap in the gup-fast path. > > [1]: https://lore.kernel.org/linux-mm/a1fa7fa2-914b-366d-9902-e5b784e8428c@shipmail.org/ > Oh, nice! That would be ideal indeed, as we would skip the pgmap lookup enterily. I suppose device-dax would use pfn_t PFN_MAP while fs-dax memory device would set PFN_MAP | PFN_DEV (provided vmf_insert_pfn_{pmd,pud} calls mkspecial on PFN_DEV). I haven't been following that thread, but for PMD/PUD special in vmf_* these might be useful: https://lore.kernel.org/linux-mm/20200110190313.17144-2-joao.m.martins@oracle.com/ https://lore.kernel.org/linux-mm/20200110190313.17144-4-joao.m.martins@oracle.com/
On Thu, Mar 25, 2021 at 7:34 AM Jason Gunthorpe <jgg@ziepe.ca> wrote: > > On Thu, Mar 18, 2021 at 10:03:06AM -0700, Dan Williams wrote: > > Yes. I still need to answer the question of whether mapping > > invalidation triggers longterm pin holders to relinquish their hold, > > but that's a problem regardless of whether gup-fast is supported or > > not. > > It does not, GUP users do not interact with addres_space or mmu > notifiers > Ok, but the SIGKILL from the memory_failure() will drop the pin? Can memory_failure() find the right processes to kill if the memory registration has been passed by SCM_RIGHTS?
On 3/24/21 7:00 PM, Joao Martins wrote: > On 3/24/21 5:45 PM, Dan Williams wrote: >> On Thu, Mar 18, 2021 at 3:02 AM Joao Martins <joao.m.martins@oracle.com> wrote: >>> On 3/18/21 4:08 AM, Dan Williams wrote: >>>> Now that device-dax and filesystem-dax are guaranteed to unmap all user >>>> mappings of devmap / DAX pages before tearing down the 'struct page' >>>> array, get_user_pages_fast() can rely on its traditional synchronization >>>> method "validate_pte(); get_page(); revalidate_pte()" to catch races with >>>> device shutdown. Specifically the unmap guarantee ensures that gup-fast >>>> either succeeds in taking a page reference (lock-less), or it detects a >>>> need to fall back to the slow path where the device presence can be >>>> revalidated with locks held. >>> >>> [...] >>> >>> So for allowing FOLL_LONGTERM[0] would it be OK if we used page->pgmap after >>> try_grab_page() for checking pgmap type to see if we are in a device-dax >>> longterm pin? >> >> So, there is an effort to add a new pte bit p{m,u}d_special to disable >> gup-fast for huge pages [1]. I'd like to investigate whether we could >> use devmap + special as an encoding for "no longterm" and never >> consult the pgmap in the gup-fast path. >> >> [1]: https://lore.kernel.org/linux-mm/a1fa7fa2-914b-366d-9902-e5b784e8428c@shipmail.org/ >> > > Oh, nice! That would be ideal indeed, as we would skip the pgmap lookup enterily. > > I suppose device-dax would use pfn_t PFN_MAP while fs-dax memory device would set PFN_MAP > | PFN_DEV (provided vmf_insert_pfn_{pmd,pud} calls mkspecial on PFN_DEV). > > I haven't been following that thread, but for PMD/PUD special in vmf_* these might be useful: > > https://lore.kernel.org/linux-mm/20200110190313.17144-2-joao.m.martins@oracle.com/ > https://lore.kernel.org/linux-mm/20200110190313.17144-4-joao.m.martins@oracle.com/ > On a second thought, maybe it doesn't need to be that complicated for {fs,dev}dax if the added special bit is just a subcase of devmap pte/pmd/puds. See below scsissors mark as a rough estimation on the changes (nothing formal/proper as it isn't properly splitted). Running gup_test with devdax/fsdax FOLL_LONGTERM and without does the intended. (gup_test -m 16384 -r 10 -a -S -n 512 -w -f <file> [-F 0x10000]). Unless this is about using special PMD/PUD bits without page structs (thus without devmap bits) as in the previous two links. -- >8 -- Subject: mm/gup, nvdimm: allow FOLL_LONGTERM for device-dax gup-fast Signed-off-by: Joao Martins <joao.m.martins@oracle.com> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index 47027796c2f9..8b5d68d89cde 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -439,11 +439,16 @@ static inline pmd_t pmd_mkinvalid(pmd_t pmd) #ifdef CONFIG_TRANSPARENT_HUGEPAGE #define pmd_devmap(pmd) pte_devmap(pmd_pte(pmd)) +#define pmd_special(pmd) pte_special(pmd_pte(pmd)) #endif static inline pmd_t pmd_mkdevmap(pmd_t pmd) { return pte_pmd(set_pte_bit(pmd_pte(pmd), __pgprot(PTE_DEVMAP))); } +static inline pmd_t pmd_mkspecial(pmd_t pmd) +{ + return pte_pmd(set_pte_bit(pmd_pte(pmd), __pgprot(PTE_SPECIAL))); +} #define __pmd_to_phys(pmd) __pte_to_phys(pmd_pte(pmd)) #define __phys_to_pmd_val(phys) __phys_to_pte_val(phys) diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h index b1099f2d9800..45449ee86d4f 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -259,13 +259,13 @@ static inline int pmd_large(pmd_t pte) /* NOTE: when predicate huge page, consider also pmd_devmap, or use pmd_large */ static inline int pmd_trans_huge(pmd_t pmd) { - return (pmd_val(pmd) & (_PAGE_PSE|_PAGE_DEVMAP)) == _PAGE_PSE; + return (pmd_val(pmd) & (_PAGE_PSE|_PAGE_DEVMAP|_PAGE_SPECIAL)) == _PAGE_PSE; } #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD static inline int pud_trans_huge(pud_t pud) { - return (pud_val(pud) & (_PAGE_PSE|_PAGE_DEVMAP)) == _PAGE_PSE; + return (pud_val(pud) & (_PAGE_PSE|_PAGE_DEVMAP|_PAGE_SPECIAL)) == _PAGE_PSE; } #endif @@ -297,6 +297,19 @@ static inline int pgd_devmap(pgd_t pgd) { return 0; } + +static inline bool pmd_special(pmd_t pmd) +{ + return !!(pmd_flags(pmd) & _PAGE_SPECIAL); +} + +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD +static inline bool pud_special(pud_t pud) +{ + return !!(pud_flags(pud) & _PAGE_SPECIAL); +} +#endif + #endif #endif /* CONFIG_TRANSPARENT_HUGEPAGE */ @@ -452,6 +465,11 @@ static inline pmd_t pmd_mkdevmap(pmd_t pmd) return pmd_set_flags(pmd, _PAGE_DEVMAP); } +static inline pmd_t pmd_mkspecial(pmd_t pmd) +{ + return pmd_set_flags(pmd, _PAGE_SPECIAL); +} + static inline pmd_t pmd_mkhuge(pmd_t pmd) { return pmd_set_flags(pmd, _PAGE_PSE); @@ -511,6 +529,11 @@ static inline pud_t pud_mkhuge(pud_t pud) return pud_set_flags(pud, _PAGE_PSE); } +static inline pud_t pud_mkspecial(pud_t pud) +{ + return pud_set_flags(pud, _PAGE_SPECIAL); +} + static inline pud_t pud_mkyoung(pud_t pud) { return pud_set_flags(pud, _PAGE_ACCESSED); diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c index 16760b237229..156ceae33164 100644 --- a/drivers/nvdimm/pmem.c +++ b/drivers/nvdimm/pmem.c @@ -435,7 +435,7 @@ static int pmem_attach_disk(struct device *dev, pmem->data_offset = le64_to_cpu(pfn_sb->dataoff); pmem->pfn_pad = resource_size(res) - range_len(&pmem->pgmap.range); - pmem->pfn_flags |= PFN_MAP; + pmem->pfn_flags |= PFN_MAP | PFN_SPECIAL; bb_range = pmem->pgmap.range; bb_range.start += pmem->data_offset; } else if (pmem_should_map_pages(dev)) { @@ -445,7 +445,7 @@ static int pmem_attach_disk(struct device *dev, pmem->pgmap.type = MEMORY_DEVICE_FS_DAX; pmem->pgmap.ops = &fsdax_pagemap_ops; addr = devm_memremap_pages(dev, &pmem->pgmap); - pmem->pfn_flags |= PFN_MAP; + pmem->pfn_flags |= PFN_MAP | PFN_SPECIAL; bb_range = pmem->pgmap.range; } else { if (devm_add_action_or_reset(dev, pmem_release_queue, diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c index 4ee6f734ba83..873c8e53c85d 100644 --- a/fs/fuse/virtio_fs.c +++ b/fs/fuse/virtio_fs.c @@ -748,7 +748,7 @@ static long virtio_fs_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, *kaddr = fs->window_kaddr + offset; if (pfn) *pfn = phys_to_pfn_t(fs->window_phys_addr + offset, - PFN_DEV | PFN_MAP); + PFN_DEV | PFN_MAP | PFN_SPECIAL); return nr_pages > max_nr_pages ? max_nr_pages : nr_pages; } diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h index 7364e5a70228..ad7078e38ef2 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -1189,6 +1189,18 @@ static inline int pgd_devmap(pgd_t pgd) } #endif +#if !defined(CONFIG_ARCH_HAS_PTE_SPECIAL) || !defined(CONFIG_TRANSPARENT_HUGEPAGE) +static inline bool pmd_special(pmd_t pmd) +{ + return false; +} + +static inline pmd_t pmd_mkspecial(pmd_t pmd) +{ + return pmd; +} +#endif + #if !defined(CONFIG_TRANSPARENT_HUGEPAGE) || \ (defined(CONFIG_TRANSPARENT_HUGEPAGE) && \ !defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)) @@ -1196,6 +1208,14 @@ static inline int pud_trans_huge(pud_t pud) { return 0; } +static inline bool pud_special(pud_t pud) +{ + return false; +} +static inline pud_t pud_mkspecial(pud_t pud) +{ + return pud; +} #endif /* See pmd_none_or_trans_huge_or_clear_bad for discussion. */ diff --git a/mm/gup.c b/mm/gup.c index b3e647c8b7ee..87aa229a9347 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2086,7 +2086,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, goto pte_unmap; if (pte_devmap(pte)) { - if (unlikely(flags & FOLL_LONGTERM)) + if (unlikely(flags & FOLL_LONGTERM) && pte_special(pte)) goto pte_unmap; pgmap = get_dev_pagemap(pte_pfn(pte), pgmap); @@ -2338,7 +2338,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr, return 0; if (pmd_devmap(orig)) { - if (unlikely(flags & FOLL_LONGTERM)) + if (unlikely(flags & FOLL_LONGTERM) && pmd_special(orig)) return 0; return __gup_device_huge_pmd(orig, pmdp, addr, end, flags, pages, nr); @@ -2372,7 +2372,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr, return 0; if (pud_devmap(orig)) { - if (unlikely(flags & FOLL_LONGTERM)) + if (unlikely(flags & FOLL_LONGTERM) && pud_special(orig)) return 0; return __gup_device_huge_pud(orig, pudp, addr, end, flags, pages, nr); diff --git a/mm/huge_memory.c b/mm/huge_memory.c index f6f70632fc29..9d5117711919 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -796,8 +796,11 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr, } entry = pmd_mkhuge(pfn_t_pmd(pfn, prot)); - if (pfn_t_devmap(pfn)) + if (pfn_t_devmap(pfn)) { entry = pmd_mkdevmap(entry); + if (pfn_t_special(pfn)) + entry = pmd_mkspecial(entry); + } if (write) { entry = pmd_mkyoung(pmd_mkdirty(entry)); entry = maybe_pmd_mkwrite(entry, vma); @@ -896,8 +899,11 @@ static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr, } entry = pud_mkhuge(pfn_t_pud(pfn, prot)); - if (pfn_t_devmap(pfn)) + if (pfn_t_devmap(pfn)) { entry = pud_mkdevmap(entry); + if (pfn_t_special(pfn)) + entry = pud_mkspecial(entry); + } if (write) { entry = pud_mkyoung(pud_mkdirty(entry)); entry = maybe_pud_mkwrite(entry, vma);
diff --git a/mm/gup.c b/mm/gup.c index e40579624f10..dfeb47e4e8d4 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1996,9 +1996,8 @@ static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start, static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, unsigned int flags, struct page **pages, int *nr) { - struct dev_pagemap *pgmap = NULL; - int nr_start = *nr, ret = 0; pte_t *ptep, *ptem; + int ret = 0; ptem = ptep = pte_offset_map(&pmd, addr); do { @@ -2015,16 +2014,10 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, if (!pte_access_permitted(pte, flags & FOLL_WRITE)) goto pte_unmap; - if (pte_devmap(pte)) { - if (unlikely(flags & FOLL_LONGTERM)) - goto pte_unmap; + if (pte_devmap(pte) && (flags & FOLL_LONGTERM)) + goto pte_unmap; - pgmap = get_dev_pagemap(pte_pfn(pte), pgmap); - if (unlikely(!pgmap)) { - undo_dev_pagemap(nr, nr_start, flags, pages); - goto pte_unmap; - } - } else if (pte_special(pte)) + if (pte_special(pte)) goto pte_unmap; VM_BUG_ON(!pfn_valid(pte_pfn(pte))); @@ -2063,8 +2056,6 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, ret = 1; pte_unmap: - if (pgmap) - put_dev_pagemap(pgmap); pte_unmap(ptem); return ret; } @@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */ #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE) + static int __gup_device_huge(unsigned long pfn, unsigned long addr, unsigned long end, unsigned int flags, struct page **pages, int *nr) { int nr_start = *nr; - struct dev_pagemap *pgmap = NULL; do { - struct page *page = pfn_to_page(pfn); + struct page *page; + + /* + * Typically pfn_to_page() on a devmap pfn is not safe + * without holding a live reference on the hosting + * pgmap. In the gup-fast path it is safe because any + * races will be resolved by either gup-fast taking a + * reference or the shutdown path unmapping the pte to + * trigger gup-fast to fall back to the slow path. + */ + page = pfn_to_page(pfn); - pgmap = get_dev_pagemap(pfn, pgmap); - if (unlikely(!pgmap)) { - undo_dev_pagemap(nr, nr_start, flags, pages); - return 0; - } SetPageReferenced(page); pages[*nr] = page; if (unlikely(!try_grab_page(page, flags))) { @@ -2112,8 +2108,6 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr, pfn++; } while (addr += PAGE_SIZE, addr != end); - if (pgmap) - put_dev_pagemap(pgmap); return 1; }
Now that device-dax and filesystem-dax are guaranteed to unmap all user mappings of devmap / DAX pages before tearing down the 'struct page' array, get_user_pages_fast() can rely on its traditional synchronization method "validate_pte(); get_page(); revalidate_pte()" to catch races with device shutdown. Specifically the unmap guarantee ensures that gup-fast either succeeds in taking a page reference (lock-less), or it detects a need to fall back to the slow path where the device presence can be revalidated with locks held. Reported-by: Jason Gunthorpe <jgg@ziepe.ca> Cc: Christoph Hellwig <hch@lst.de> Cc: Shiyang Ruan <ruansy.fnst@fujitsu.com> Cc: Vishal Verma <vishal.l.verma@intel.com> Cc: Dave Jiang <dave.jiang@intel.com> Cc: Ira Weiny <ira.weiny@intel.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Jan Kara <jack@suse.cz> Cc: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Dan Williams <dan.j.williams@intel.com> --- mm/gup.c | 38 ++++++++++++++++---------------------- 1 file changed, 16 insertions(+), 22 deletions(-)