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Date: Fri, 27 May 2022 11:28:15 +1000
From: Alistair Popple <apopple@...dia.com>
To: "Sierra Guiza, Alejandro (Alex)" <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 v3 02/13] mm: handling Non-LRU pages returned by
vm_normal_pages
"Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com> writes:
> 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:
Sorry, I must not have been clear enough. My understanding is if the
following condition is true:
>> if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) {
Then follow_page_pte() could return a non-LRU page even when FOLL_LRU is
specified (because I think a devmap page is a non-LRU page). That seems
confusing, so for consistency I was suggesting we should not return
devmap pages for FOLL_LRU.
To be clear I don't think there is an actual problem here atm, but the
inconsistency could easily lead to one in future.
> 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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