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Message-ID: <87wnech1as.fsf@nvdebian.thelocal>
Date: Mon, 23 May 2022 22:02:35 +1000
From: Alistair Popple <apopple@...dia.com>
To: Alex Sierra <alex.sierra@....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 v2 11/13] mm: handling Non-LRU pages returned by
vm_normal_pages
Technically I think this patch should be earlier in the series. As I
understand it patch 1 allows DEVICE_COHERENT pages to be inserted in the
page tables and therefore makes it possible for page table walkers to
see non-LRU pages.
Some more comments below:
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.
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?
[...]
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index a4e5eaf3eb01..eb3cfd679800 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 (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 (page && is_zone_device_page(page))
> + goto abort;
Are either of these two cases actually possible? DEVICE_COHERENT doesn't
currently support THP, so if I'm understanding correctly we couldn't
have a pte mapped DEVICE_COHERENT THP right? Assuming that's the case I
think WARN_ON_ONCE() would be better.
Otherwise I think everything else looks reasonable.
> 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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