Re: [PATCH v3 02/13] mm: handling Non-LRU pages returned by vm_normal_pages

[Alistair Popple <apopple@nvidia.com>]

Alex Sierra <alex.sierra@amd.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)))

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@amd.com>
> Acked-by: Felix Kuehling <Felix.Kuehling@amd.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 */