Date:   Thu, 23 Jun 2022 13:20:45 -0500
From:   "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>
To:     David Hildenbrand <david@...hat.com>,
        Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org
Cc:     Felix Kuehling <felix.kuehling@....com>, jgg@...dia.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
Subject: Re: [PATCH v5 01/13] mm: add zone device coherent type memory support


On 6/23/2022 2:57 AM, David Hildenbrand wrote:
> On 23.06.22 01:16, Sierra Guiza, Alejandro (Alex) wrote:
>> On 6/21/2022 11:16 AM, David Hildenbrand wrote:
>>> On 21.06.22 18:08, Sierra Guiza, Alejandro (Alex) wrote:
>>>> On 6/21/2022 7:25 AM, David Hildenbrand wrote:
>>>>> On 21.06.22 13:55, Alistair Popple wrote:
>>>>>> David Hildenbrand<david@...hat.com>  writes:
>>>>>>
>>>>>>> On 21.06.22 13:25, Felix Kuehling wrote:
>>>>>>>> Am 6/17/22 um 23:19 schrieb David Hildenbrand:
>>>>>>>>> On 17.06.22 21:27, Sierra Guiza, Alejandro (Alex) wrote:
>>>>>>>>>> On 6/17/2022 12:33 PM, David Hildenbrand wrote:
>>>>>>>>>>> On 17.06.22 19:20, Sierra Guiza, Alejandro (Alex) wrote:
>>>>>>>>>>>> On 6/17/2022 4:40 AM, David Hildenbrand wrote:
>>>>>>>>>>>>> On 31.05.22 22:00, Alex Sierra wrote:
>>>>>>>>>>>>>> Device memory that is cache coherent from device and CPU point of view.
>>>>>>>>>>>>>> This is used on platforms that have an advanced system bus (like CAPI
>>>>>>>>>>>>>> or CXL). Any page of a process can be migrated to such memory. However,
>>>>>>>>>>>>>> no one should be allowed to pin such memory so that it can always be
>>>>>>>>>>>>>> evicted.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Signed-off-by: Alex Sierra<alex.sierra@....com>
>>>>>>>>>>>>>> Acked-by: Felix Kuehling<Felix.Kuehling@....com>
>>>>>>>>>>>>>> Reviewed-by: Alistair Popple<apopple@...dia.com>
>>>>>>>>>>>>>> [hch: rebased ontop of the refcount changes,
>>>>>>>>>>>>>>             removed is_dev_private_or_coherent_page]
>>>>>>>>>>>>>> Signed-off-by: Christoph Hellwig<hch@....de>
>>>>>>>>>>>>>> ---
>>>>>>>>>>>>>>        include/linux/memremap.h | 19 +++++++++++++++++++
>>>>>>>>>>>>>>        mm/memcontrol.c          |  7 ++++---
>>>>>>>>>>>>>>        mm/memory-failure.c      |  8 ++++++--
>>>>>>>>>>>>>>        mm/memremap.c            | 10 ++++++++++
>>>>>>>>>>>>>>        mm/migrate_device.c      | 16 +++++++---------
>>>>>>>>>>>>>>        mm/rmap.c                |  5 +++--
>>>>>>>>>>>>>>        6 files changed, 49 insertions(+), 16 deletions(-)
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>>>>>>>>>>>>>> index 8af304f6b504..9f752ebed613 100644
>>>>>>>>>>>>>> --- a/include/linux/memremap.h
>>>>>>>>>>>>>> +++ b/include/linux/memremap.h
>>>>>>>>>>>>>> @@ -41,6 +41,13 @@ struct vmem_altmap {
>>>>>>>>>>>>>>         * A more complete discussion of unaddressable memory may be found in
>>>>>>>>>>>>>>         * include/linux/hmm.h and Documentation/vm/hmm.rst.
>>>>>>>>>>>>>>         *
>>>>>>>>>>>>>> + * MEMORY_DEVICE_COHERENT:
>>>>>>>>>>>>>> + * Device memory that is cache coherent from device and CPU point of view. This
>>>>>>>>>>>>>> + * is used on platforms that have an advanced system bus (like CAPI or CXL). A
>>>>>>>>>>>>>> + * driver can hotplug the device memory using ZONE_DEVICE and with that memory
>>>>>>>>>>>>>> + * type. Any page of a process can be migrated to such memory. However no one
>>>>>>>>>>>>> Any page might not be right, I'm pretty sure. ... just thinking about special pages
>>>>>>>>>>>>> like vdso, shared zeropage, ... pinned pages ...
>>>>>>>>>>> Well, you cannot migrate long term pages, that's what I meant :)
>>>>>>>>>>>
>>>>>>>>>>>>>> + * should be allowed to pin such memory so that it can always be evicted.
>>>>>>>>>>>>>> + *
>>>>>>>>>>>>>>         * MEMORY_DEVICE_FS_DAX:
>>>>>>>>>>>>>>         * Host memory that has similar access semantics as System RAM i.e. DMA
>>>>>>>>>>>>>>         * coherent and supports page pinning. In support of coordinating page
>>>>>>>>>>>>>> @@ -61,6 +68,7 @@ struct vmem_altmap {
>>>>>>>>>>>>>>        enum memory_type {
>>>>>>>>>>>>>>        	/* 0 is reserved to catch uninitialized type fields */
>>>>>>>>>>>>>>        	MEMORY_DEVICE_PRIVATE = 1,
>>>>>>>>>>>>>> +	MEMORY_DEVICE_COHERENT,
>>>>>>>>>>>>>>        	MEMORY_DEVICE_FS_DAX,
>>>>>>>>>>>>>>        	MEMORY_DEVICE_GENERIC,
>>>>>>>>>>>>>>        	MEMORY_DEVICE_PCI_P2PDMA,
>>>>>>>>>>>>>> @@ -143,6 +151,17 @@ static inline bool folio_is_device_private(const struct folio *folio)
>>>>>>>>>>>>> In general, this LGTM, and it should be correct with PageAnonExclusive I think.
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> However, where exactly is pinning forbidden?
>>>>>>>>>>>> Long-term pinning is forbidden since it would interfere with the device
>>>>>>>>>>>> memory manager owning the
>>>>>>>>>>>> device-coherent pages (e.g. evictions in TTM). However, normal pinning
>>>>>>>>>>>> is allowed on this device type.
>>>>>>>>>>> I don't see updates to folio_is_pinnable() in this patch.
>>>>>>>>>> Device coherent type pages should return true here, as they are pinnable
>>>>>>>>>> pages.
>>>>>>>>> That function is only called for long-term pinnings in try_grab_folio().
>>>>>>>>>
>>>>>>>>>>> So wouldn't try_grab_folio() simply pin these pages? What am I missing?
>>>>>>>>>> As far as I understand this return NULL for long term pin pages.
>>>>>>>>>> Otherwise they get refcount incremented.
>>>>>>>>> I don't follow.
>>>>>>>>>
>>>>>>>>> You're saying
>>>>>>>>>
>>>>>>>>> a) folio_is_pinnable() returns true for device coherent pages
>>>>>>>>>
>>>>>>>>> and that
>>>>>>>>>
>>>>>>>>> b) device coherent pages don't get long-term pinned
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Yet, the code says
>>>>>>>>>
>>>>>>>>> struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
>>>>>>>>> {
>>>>>>>>> 	if (flags & FOLL_GET)
>>>>>>>>> 		return try_get_folio(page, refs);
>>>>>>>>> 	else if (flags & FOLL_PIN) {
>>>>>>>>> 		struct folio *folio;
>>>>>>>>>
>>>>>>>>> 		/*
>>>>>>>>> 		 * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
>>>>>>>>> 		 * right zone, so fail and let the caller fall back to the slow
>>>>>>>>> 		 * path.
>>>>>>>>> 		 */
>>>>>>>>> 		if (unlikely((flags & FOLL_LONGTERM) &&
>>>>>>>>> 			     !is_pinnable_page(page)))
>>>>>>>>> 			return NULL;
>>>>>>>>> 		...
>>>>>>>>> 		return folio;
>>>>>>>>> 	}
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> What prevents these pages from getting long-term pinned as stated in this patch?
>>>>>>>> Long-term pinning is handled by __gup_longterm_locked, which migrates
>>>>>>>> pages returned by __get_user_pages_locked that cannot be long-term
>>>>>>>> pinned. try_grab_folio is OK to grab the pages. Anything that can't be
>>>>>>>> long-term pinned will be migrated afterwards, and
>>>>>>>> __get_user_pages_locked will be retried. The migration of
>>>>>>>> DEVICE_COHERENT pages was implemented by Alistair in patch 5/13
>>>>>>>> ("mm/gup: migrate device coherent pages when pinning instead of failing").
>>>>>>> Thanks.
>>>>>>>
>>>>>>> __gup_longterm_locked()->check_and_migrate_movable_pages()
>>>>>>>
>>>>>>> Which checks folio_is_pinnable() and doesn't do anything if set.
>>>>>>>
>>>>>>> Sorry to be dense here, but I don't see how what's stated in this patch
>>>>>>> works without adjusting folio_is_pinnable().
>>>>>> Ugh, I think you might be right about try_grab_folio().
>>>>>>
>>>>>> We didn't update folio_is_pinnable() to include device coherent pages
>>>>>> because device coherent pages are pinnable. It is really just
>>>>>> FOLL_LONGTERM that we want to prevent here.
>>>>>>
>>>>>> For normal PUP that is done by my change in
>>>>>> check_and_migrate_movable_pages() which migrates pages being pinned with
>>>>>> FOLL_LONGTERM. But I think I incorrectly assumed we would take the
>>>>>> pte_devmap() path in gup_pte_range(), which we don't for coherent pages.
>>>>>> So I think the check in try_grab_folio() needs to be:
>>>>> I think I said it already (and I might be wrong without reading the
>>>>> code), but folio_is_pinnable() is *only* called for long-term pinnings.
>>>>>
>>>>> It should actually be called folio_is_longterm_pinnable().
>>>>>
>>>>> That's where that check should go, no?
>>>> David, I think you're right. We didn't catch this since the LONGTERM gup
>>>> test we added to hmm-test only calls to pin_user_pages. Apparently
>>>> try_grab_folio is called only from fast callers (ex.
>>>> pin_user_pages_fast/get_user_pages_fast). I have added a conditional
>>>> similar to what Alistair has proposed to return null on LONGTERM &&
>>>> (coherent_pages || folio_is_pinnable) at try_grab_folio. Also a new gup
>>>> test was added with LONGTERM set that calls pin_user_pages_fast.
>>>> Returning null under this condition it does causes the migration from
>>>> dev to system memory.
>>>>
>>> Why can't coherent memory simply put its checks into
>>> folio_is_pinnable()? I don't get it why we have to do things differently
>>> here.
>>>
>>>> Actually, Im having different problems with a call to PageAnonExclusive
>>>> from try_to_migrate_one during page fault from a HMM test that first
>>>> migrate pages to device private and forks to mark as COW these pages.
>>>> Apparently is catching the first BUG VM_BUG_ON_PGFLAGS(!PageAnon(page),
>>>> page)
>>> With or without this series? A backtrace would be great.
>> Here's the back trace. This happens in a hmm-test added in this patch
>> series. However, I have tried to isolate this BUG by just adding the COW
>> test with private device memory only. This is only present as follows.
>> Allocate anonymous mem->Migrate to private device memory->fork->try to
>> access to parent's anonymous memory (which will suppose to trigger a
>> page fault and migration to system mem). Just for the record, if the
>> child is terminated before the parent's memory is accessed, this problem
>> is not present.
>
> The only usage of PageAnonExclusive() in try_to_migrate_one() is:
>
> anon_exclusive = folio_test_anon(folio) &&
> 		 PageAnonExclusive(subpage);
>
> Which can only possibly fail if subpage is not actually part of the folio.
>
>
> I see some controversial code in the the if (folio_is_zone_device(folio)) case later:
>
> 			 * The assignment to subpage above was computed from a
> 			 * swap PTE which results in an invalid pointer.
> 			 * Since only PAGE_SIZE pages can currently be
> 			 * migrated, just set it to page. This will need to be
> 			 * changed when hugepage migrations to device private
> 			 * memory are supported.
> 			 */
> 			subpage = &folio->page;
>
> There we have our invalid pointer hint.
>
> I don't see how it could have worked if the child quit, though? Maybe
> just pure luck?
>
>
> Does the following fix your issue:

Yes, it fixed the issue. Thanks. Should we include this patch in this 
patch series or separated?

Regards,
Alex Sierra
>
>
>
>  From 09750c714739ef3ca317b4aec82bf20283c8fd2d Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <david@...hat.com>
> Date: Thu, 23 Jun 2022 09:38:45 +0200
> Subject: [PATCH] mm/rmap: fix dereferencing invalid subpage pointer in
>   try_to_migrate_one()
>
> The subpage we calculate is an invalid pointer for device private pages,
> because device private pages are mapped via non-present device private
> entries, not ordinary present PTEs.
>
> Let's just not compute broken pointers and fixup later. Move the proper
> assignment of the correct subpage to the beginning of the function and
> assert that we really only have a single page in our folio.
>
> This currently results in a BUG when tying to compute anon_exclusive,
> because:
>
> [  528.727237] BUG: unable to handle page fault for address: ffffea1fffffffc0
> [  528.739585] #PF: supervisor read access in kernel mode
> [  528.745324] #PF: error_code(0x0000) - not-present page
> [  528.751062] PGD 44eaf2067 P4D 44eaf2067 PUD 0
> [  528.756026] Oops: 0000 [#1] PREEMPT SMP NOPTI
> [  528.760890] CPU: 120 PID: 18275 Comm: hmm-tests Not tainted 5.19.0-rc3-kfd-alex #257
> [  528.769542] Hardware name: AMD Corporation BardPeak/BardPeak, BIOS RTY1002BDS 09/17/2021
> [  528.778579] RIP: 0010:try_to_migrate_one+0x21a/0x1000
> [  528.784225] Code: f6 48 89 c8 48 2b 05 45 d1 6a 01 48 c1 f8 06 48 29
> c3 48 8b 45 a8 48 c1 e3 06 48 01 cb f6 41 18 01 48 89 85 50 ff ff ff 74
> 0b <4c> 8b 33 49 c1 ee 11 41 83 e6 01 48 8b bd 48 ff ff ff e8 3f 99 02
> [  528.805194] RSP: 0000:ffffc90003cdfaa0 EFLAGS: 00010202
> [  528.811027] RAX: 00007ffff7ff4000 RBX: ffffea1fffffffc0 RCX: ffffeaffffffffc0
> [  528.818995] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffc90003cdfaf8
> [  528.826962] RBP: ffffc90003cdfb70 R08: 0000000000000000 R09: 0000000000000000
> [  528.834930] R10: ffffc90003cdf910 R11: 0000000000000002 R12: ffff888194450540
> [  528.842899] R13: ffff888160d057c0 R14: 0000000000000000 R15: 03ffffffffffffff
> [  528.850865] FS:  00007ffff7fdb740(0000) GS:ffff8883b0600000(0000) knlGS:0000000000000000
> [  528.859891] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  528.866308] CR2: ffffea1fffffffc0 CR3: 00000001562b4003 CR4: 0000000000770ee0
> [  528.874275] PKRU: 55555554
> [  528.877286] Call Trace:
> [  528.880016]  <TASK>
> [  528.882356]  ? lock_is_held_type+0xdf/0x130
> [  528.887033]  rmap_walk_anon+0x167/0x410
> [  528.891316]  try_to_migrate+0x90/0xd0
> [  528.895405]  ? try_to_unmap_one+0xe10/0xe10
> [  528.900074]  ? anon_vma_ctor+0x50/0x50
> [  528.904260]  ? put_anon_vma+0x10/0x10
> [  528.908347]  ? invalid_mkclean_vma+0x20/0x20
> [  528.913114]  migrate_vma_setup+0x5f4/0x750
> [  528.917691]  dmirror_devmem_fault+0x8c/0x250 [test_hmm]
> [  528.923532]  do_swap_page+0xac0/0xe50
> [  528.927623]  ? __lock_acquire+0x4b2/0x1ac0
> [  528.932199]  __handle_mm_fault+0x949/0x1440
> [  528.936876]  handle_mm_fault+0x13f/0x3e0
> [  528.941256]  do_user_addr_fault+0x215/0x740
> [  528.945928]  exc_page_fault+0x75/0x280
> [  528.950115]  asm_exc_page_fault+0x27/0x30
> [  528.954593] RIP: 0033:0x40366b
> ...
>
> Fixes: 6c287605fd56 ("mm: remember exclusively mapped anonymous pages with PG_anon_exclusive")
> Reported-by: Sierra Guiza, Alejandro (Alex) <alex.sierra@....com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Christoph Hellwig <hch@....de>
> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>   mm/rmap.c | 27 +++++++++++++++++----------
>   1 file changed, 17 insertions(+), 10 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5bcb334cd6f2..746c05acad27 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1899,8 +1899,23 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>   		/* Unexpected PMD-mapped THP? */
>   		VM_BUG_ON_FOLIO(!pvmw.pte, folio);
>   
> -		subpage = folio_page(folio,
> -				pte_pfn(*pvmw.pte) - folio_pfn(folio));
> +		if (folio_is_zone_device(folio)) {
> +			/*
> +			 * Our PTE is a non-present device exclusive entry and
> +			 * calculating the subpage as for the common case would
> +			 * result in an invalid pointer.
> +			 *
> +			 * Since only PAGE_SIZE pages can currently be
> +			 * migrated, just set it to page. This will need to be
> +			 * changed when hugepage migrations to device private
> +			 * memory are supported.
> +			 */
> +			VM_BUG_ON_FOLIO(folio_nr_pages(folio) > 1, folio);
> +			subpage = &folio->page;
> +		} else {
> +			subpage = folio_page(folio,
> +					pte_pfn(*pvmw.pte) - folio_pfn(folio));
> +		}
>   		address = pvmw.address;
>   		anon_exclusive = folio_test_anon(folio) &&
>   				 PageAnonExclusive(subpage);
> @@ -1993,15 +2008,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>   			/*
>   			 * No need to invalidate here it will synchronize on
>   			 * against the special swap migration pte.
> -			 *
> -			 * The assignment to subpage above was computed from a
> -			 * swap PTE which results in an invalid pointer.
> -			 * Since only PAGE_SIZE pages can currently be
> -			 * migrated, just set it to page. This will need to be
> -			 * changed when hugepage migrations to device private
> -			 * memory are supported.
>   			 */
> -			subpage = &folio->page;
>   		} else if (PageHWPoison(subpage)) {
>   			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
>   			if (folio_test_hugetlb(folio)) {

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