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Date: Thu, 26 May 2022 11:07:47 -0500 From: "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com> To: Alistair Popple <apopple@...dia.com> Cc: jgg@...dia.com, david@...hat.com, Felix.Kuehling@....com, linux-mm@...ck.org, rcampbell@...dia.com, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org, hch@....de, jglisse@...hat.com, willy@...radead.org, akpm@...ux-foundation.org Subject: Re: [PATCH v3 02/13] mm: handling Non-LRU pages returned by vm_normal_pages On 5/24/2022 11:11 PM, Alistair Popple wrote: > Alex Sierra <alex.sierra@....com> writes: > >> With DEVICE_COHERENT, we'll soon have vm_normal_pages() return >> device-managed anonymous pages that are not LRU pages. Although they >> behave like normal pages for purposes of mapping in CPU page, and for >> COW. They do not support LRU lists, NUMA migration or THP. >> >> We also introduced a FOLL_LRU flag that adds the same behaviour to >> follow_page and related APIs, to allow callers to specify that they >> expect to put pages on an LRU list. > Continuing the follow up from the thread for v2: > >>> This means by default GUP can return non-LRU pages. I didn't see >>> anywhere that would be a problem but I didn't check everything. Did you >>> check this or is there some other reason I've missed that makes this not >>> a problem? >> I have double checked all gup and pin_user_pages callers and none of them seem >> to have interaction with LRU APIs. > And actually if I'm understanding things correctly callers of > GUP/PUP/follow_page_pte() should already expect to get non-LRU pages > returned: > > page = vm_normal_page(vma, address, pte); > if ((flags & FOLL_LRU) && page && is_zone_device_page(page)) > page = NULL; > if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) { > /* > * Only return device mapping pages in the FOLL_GET or FOLL_PIN > * case since they are only valid while holding the pgmap > * reference. > */ > *pgmap = get_dev_pagemap(pte_pfn(pte), *pgmap); > if (*pgmap) > page = pte_page(pte); > > Which I think makes FOLL_LRU confusing, because if understand correctly > even with FOLL_LRU it is still possible for follow_page_pte() to return > a non-LRU page. Could we do something like this to make it consistent: > > if ((flags & FOLL_LRU) && (page && is_zone_device_page(page) || > !page && pte_devmap(pte))) Hi Alistair, Not sure if this suggestion is a replacement for the first or the second condition in the snip code above. We know device coherent type will not be set with devmap. So we could do the following: if ((flags & FOLL_LRU) && page && is_zone_device_page(page)) - page = NULL; + goto no_page; Regards, Alex Sierra > > Looking at callers that currently use FOLL_LRU I don't think this would > change any behaviour as they already filter out devmap through various > other means. > >> Signed-off-by: Alex Sierra <alex.sierra@....com> >> Acked-by: Felix Kuehling <Felix.Kuehling@....com> >> --- >> fs/proc/task_mmu.c | 2 +- >> include/linux/mm.h | 3 ++- >> mm/gup.c | 2 ++ >> mm/huge_memory.c | 2 +- >> mm/khugepaged.c | 9 ++++++--- >> mm/ksm.c | 6 +++--- >> mm/madvise.c | 4 ++-- >> mm/memory.c | 9 ++++++++- >> mm/mempolicy.c | 2 +- >> mm/migrate.c | 4 ++-- >> mm/mlock.c | 2 +- >> mm/mprotect.c | 2 +- >> 12 files changed, 30 insertions(+), 17 deletions(-) >> >> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c >> index f46060eb91b5..5d620733f173 100644 >> --- a/fs/proc/task_mmu.c >> +++ b/fs/proc/task_mmu.c >> @@ -1785,7 +1785,7 @@ static struct page *can_gather_numa_stats(pte_t pte, struct vm_area_struct *vma, >> return NULL; >> >> page = vm_normal_page(vma, addr, pte); >> - if (!page) >> + if (!page || is_zone_device_page(page)) >> return NULL; >> >> if (PageReserved(page)) >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index 9f44254af8ce..d7f253a0c41e 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -601,7 +601,7 @@ struct vm_operations_struct { >> #endif >> /* >> * Called by vm_normal_page() for special PTEs to find the >> - * page for @addr. This is useful if the default behavior >> + * page for @addr. This is useful if the default behavior >> * (using pte_page()) would not find the correct page. >> */ >> struct page *(*find_special_page)(struct vm_area_struct *vma, >> @@ -2929,6 +2929,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address, >> #define FOLL_NUMA 0x200 /* force NUMA hinting page fault */ >> #define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */ >> #define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */ >> +#define FOLL_LRU 0x1000 /* return only LRU (anon or page cache) */ >> #define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */ >> #define FOLL_COW 0x4000 /* internal GUP flag */ >> #define FOLL_ANON 0x8000 /* don't do file mappings */ >> diff --git a/mm/gup.c b/mm/gup.c >> index 501bc150792c..c9cbac06bcc5 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -479,6 +479,8 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, >> } >> >> page = vm_normal_page(vma, address, pte); >> + if ((flags & FOLL_LRU) && page && is_zone_device_page(page)) >> + page = NULL; >> if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) { >> /* >> * Only return device mapping pages in the FOLL_GET or FOLL_PIN >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 910a138e9859..eed80696c5fd 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -2856,7 +2856,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start, >> } >> >> /* FOLL_DUMP to ignore special (like zero) pages */ >> - page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP); >> + page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP | FOLL_LRU); >> >> if (IS_ERR(page)) >> continue; >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index a4e5eaf3eb01..8bf4126b6b9c 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -627,7 +627,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, >> goto out; >> } >> page = vm_normal_page(vma, address, pteval); >> - if (unlikely(!page)) { >> + if (unlikely(!page) || unlikely(is_zone_device_page(page))) { >> result = SCAN_PAGE_NULL; >> goto out; >> } >> @@ -1276,7 +1276,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, >> writable = true; >> >> page = vm_normal_page(vma, _address, pteval); >> - if (unlikely(!page)) { >> + if (unlikely(!page) || unlikely(is_zone_device_page(page))) { >> result = SCAN_PAGE_NULL; >> goto out_unmap; >> } >> @@ -1484,7 +1484,8 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr) >> goto abort; >> >> page = vm_normal_page(vma, addr, *pte); >> - >> + if (WARN_ON_ONCE(page && is_zone_device_page(page))) >> + page = NULL; >> /* >> * Note that uprobe, debugger, or MAP_PRIVATE may change the >> * page table, but the new page will not be a subpage of hpage. >> @@ -1502,6 +1503,8 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr) >> if (pte_none(*pte)) >> continue; >> page = vm_normal_page(vma, addr, *pte); >> + if (WARN_ON_ONCE(page && is_zone_device_page(page))) >> + goto abort; >> page_remove_rmap(page, vma, false); >> } >> >> diff --git a/mm/ksm.c b/mm/ksm.c >> index 063a48eeb5ee..f16056efca21 100644 >> --- a/mm/ksm.c >> +++ b/mm/ksm.c >> @@ -474,7 +474,7 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr) >> do { >> cond_resched(); >> page = follow_page(vma, addr, >> - FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE); >> + FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE | FOLL_LRU); >> if (IS_ERR_OR_NULL(page)) >> break; >> if (PageKsm(page)) >> @@ -559,7 +559,7 @@ static struct page *get_mergeable_page(struct rmap_item *rmap_item) >> if (!vma) >> goto out; >> >> - page = follow_page(vma, addr, FOLL_GET); >> + page = follow_page(vma, addr, FOLL_GET | FOLL_LRU); >> if (IS_ERR_OR_NULL(page)) >> goto out; >> if (PageAnon(page)) { >> @@ -2288,7 +2288,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page) >> while (ksm_scan.address < vma->vm_end) { >> if (ksm_test_exit(mm)) >> break; >> - *page = follow_page(vma, ksm_scan.address, FOLL_GET); >> + *page = follow_page(vma, ksm_scan.address, FOLL_GET | FOLL_LRU); >> if (IS_ERR_OR_NULL(*page)) { >> ksm_scan.address += PAGE_SIZE; >> cond_resched(); >> diff --git a/mm/madvise.c b/mm/madvise.c >> index 1873616a37d2..e9c24c834e98 100644 >> --- a/mm/madvise.c >> +++ b/mm/madvise.c >> @@ -413,7 +413,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, >> continue; >> >> page = vm_normal_page(vma, addr, ptent); >> - if (!page) >> + if (!page || is_zone_device_page(page)) >> continue; >> >> /* >> @@ -628,7 +628,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr, >> } >> >> page = vm_normal_page(vma, addr, ptent); >> - if (!page) >> + if (!page || is_zone_device_page(page)) >> continue; >> >> /* >> diff --git a/mm/memory.c b/mm/memory.c >> index 76e3af9639d9..571a26805ee1 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -621,6 +621,13 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr, >> if (is_zero_pfn(pfn)) >> return NULL; >> if (pte_devmap(pte)) >> +/* >> + * NOTE: New uers of ZONE_DEVICE will not set pte_devmap() and will have >> + * refcounts incremented on their struct pages when they are inserted into >> + * PTEs, thus they are safe to return here. Legacy ZONE_DEVICE pages that set >> + * pte_devmap() do not have refcounts. Example of legacy ZONE_DEVICE is >> + * MEMORY_DEVICE_FS_DAX type in pmem or virtio_fs drivers. >> + */ >> return NULL; >> >> print_bad_pte(vma, addr, pte, NULL); >> @@ -4422,7 +4429,7 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) >> pte = pte_modify(old_pte, vma->vm_page_prot); >> >> page = vm_normal_page(vma, vmf->address, pte); >> - if (!page) >> + if (!page || is_zone_device_page(page)) >> goto out_map; >> >> /* TODO: handle PTE-mapped THP */ >> diff --git a/mm/mempolicy.c b/mm/mempolicy.c >> index 8c74107a2b15..e32edbecb0cd 100644 >> --- a/mm/mempolicy.c >> +++ b/mm/mempolicy.c >> @@ -528,7 +528,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr, >> if (!pte_present(*pte)) >> continue; >> page = vm_normal_page(vma, addr, *pte); >> - if (!page) >> + if (!page || is_zone_device_page(page)) >> continue; >> /* >> * vm_normal_page() filters out zero pages, but there might >> diff --git a/mm/migrate.c b/mm/migrate.c >> index 6c31ee1e1c9b..c5d50e96ecd7 100644 >> --- a/mm/migrate.c >> +++ b/mm/migrate.c >> @@ -1611,7 +1611,7 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr, >> goto out; >> >> /* FOLL_DUMP to ignore special (like zero) pages */ >> - page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP); >> + page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP | FOLL_LRU); >> >> err = PTR_ERR(page); >> if (IS_ERR(page)) >> @@ -1802,7 +1802,7 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages, >> goto set_status; >> >> /* FOLL_DUMP to ignore special (like zero) pages */ >> - page = follow_page(vma, addr, FOLL_DUMP); >> + page = follow_page(vma, addr, FOLL_DUMP | FOLL_LRU); >> >> err = PTR_ERR(page); >> if (IS_ERR(page)) >> diff --git a/mm/mlock.c b/mm/mlock.c >> index 716caf851043..b14e929084cc 100644 >> --- a/mm/mlock.c >> +++ b/mm/mlock.c >> @@ -333,7 +333,7 @@ static int mlock_pte_range(pmd_t *pmd, unsigned long addr, >> if (!pte_present(*pte)) >> continue; >> page = vm_normal_page(vma, addr, *pte); >> - if (!page) >> + if (!page || is_zone_device_page(page)) >> continue; >> if (PageTransCompound(page)) >> continue; >> diff --git a/mm/mprotect.c b/mm/mprotect.c >> index b69ce7a7b2b7..a6f3587ea29a 100644 >> --- a/mm/mprotect.c >> +++ b/mm/mprotect.c >> @@ -91,7 +91,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, >> continue; >> >> page = vm_normal_page(vma, addr, oldpte); >> - if (!page || PageKsm(page)) >> + if (!page || is_zone_device_page(page) || PageKsm(page)) >> continue; >> >> /* Also skip shared copy-on-write pages */
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