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Thu, 15 Jul 2021 09:39:00 +0000 From: Tiberiu Georgescu To: Peter Xu CC: "akpm@linux-foundation.org" , "catalin.marinas@arm.com" , "peterz@infradead.org" , "chinwen.chang@mediatek.com" , "linmiaohe@huawei.com" , "jannh@google.com" , "apopple@nvidia.com" , "christian.brauner@ubuntu.com" , "ebiederm@xmission.com" , "adobriyan@gmail.com" , "songmuchun@bytedance.com" , "axboe@kernel.dk" , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , Ivan Teterevkov , Florian Schmidt , "Carl Waldspurger [C]" , Hugh Dickins , Andrea Arcangeli Subject: Re: [RFC PATCH 1/1] pagemap: report swap location for shared pages Thread-Topic: [RFC PATCH 1/1] pagemap: report swap location for shared pages Thread-Index: AQHXeMRl21VMVRbyvE6e8ETg8oYttqtCo1EAgAEln4A= Date: Thu, 15 Jul 2021 09:39:00 +0000 Message-ID: References: <20210714152426.216217-1-tiberiu.georgescu@nutanix.com> <20210714152426.216217-2-tiberiu.georgescu@nutanix.com> In-Reply-To: Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: d5757ca1-7320-4938-4c7b-08d947746047 x-ms-traffictypediagnostic: DM5PR02MB2220: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-proofpoint-crosstenant: true x-ms-oob-tlc-oobclassifiers: OLM:5236; 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dkim=pass header.d=nutanix.com header.s=proofpoint20171006 header.b=mqkQ+ZL7; spf=none (imf03.hostedemail.com: domain of tiberiu.georgescu@nutanix.com has no SPF policy when checking 148.163.155.12) smtp.mailfrom=tiberiu.georgescu@nutanix.com; dmarc=pass (policy=none) header.from=nutanix.com X-Rspamd-Server: rspam02 X-Stat-Signature: 1rpqz6oobeymxbriuxk13picdg1bpyia X-Rspamd-Queue-Id: DF3D1300C8A6 X-HE-Tag: 1626342051-877300 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: --_000_F35095D5B1B8444CAF01AA73AFCBFE42nutanixcom_ Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable On 14 Jul 2021, at 17:08, Peter Xu > wrote: On Wed, Jul 14, 2021 at 03:24:26PM +0000, Tiberiu Georgescu wrote: static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm, struct vm_area_struct *vma, unsigned long addr, pte_t pte) { u64 frame =3D 0, flags =3D 0; struct page *page =3D NULL; + if (vma->vm_flags & VM_SOFTDIRTY) + flags |=3D PM_SOFT_DIRTY; + if (pte_present(pte)) { if (pm->show_pfn) frame =3D pte_pfn(pte); @@ -1374,13 +1387,22 @@ static pagemap_entry_t pte_to_pagemap_entry(struct = pagemapread *pm, flags |=3D PM_SOFT_DIRTY; if (pte_uffd_wp(pte)) flags |=3D PM_UFFD_WP; - } else if (is_swap_pte(pte)) { + } else if (is_swap_pte(pte) || shmem_file(vma->vm_file)) { swp_entry_t entry; - if (pte_swp_soft_dirty(pte)) - flags |=3D PM_SOFT_DIRTY; - if (pte_swp_uffd_wp(pte)) - flags |=3D PM_UFFD_WP; - entry =3D pte_to_swp_entry(pte); + if (is_swap_pte(pte)) { + entry =3D pte_to_swp_entry(pte); + if (pte_swp_soft_dirty(pte)) + flags |=3D PM_SOFT_DIRTY; + if (pte_swp_uffd_wp(pte)) + flags |=3D PM_UFFD_WP; + } else { + void *xa_entry =3D get_xa_entry_at_vma_addr(vma, addr); + + if (xa_is_value(xa_entry)) + entry =3D radix_to_swp_entry(xa_entry); + else + goto out; + } if (pm->show_pfn) frame =3D swp_type(entry) | (swp_offset(entry) << MAX_SWAPFILES_SHIFT); @@ -1393,9 +1415,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pa= gemapread *pm, flags |=3D PM_FILE; if (page && page_mapcount(page) =3D=3D 1) flags |=3D PM_MMAP_EXCLUSIVE; - if (vma->vm_flags & VM_SOFTDIRTY) - flags |=3D PM_SOFT_DIRTY; IMHO moving this to the entry will only work for the initial iteration, how= ever it won't really help anything, as soft-dirty should always be used in pair = with clear_refs written with value "4" first otherwise all pages will be marked soft-dirty then the pagemap data is meaningless. After the "write 4" op VM_SOFTDIRTY will be cleared and I expect the test c= ase to see all zeros again even with the patch. Indeed, the SOFT_DIRTY bit gets cleared and does not get set when we dirty = the page and swap it out again. However, the pagemap entries are not completely zeroed out. The patch mostly deals with adding the swap frame offset on the pagemap entries of swappable, non-syncable pages, even if they are MAP_SHAR= ED. Example output post-patch, after writing 4 to clear_refs and dirtying the p= ages: $ dd if=3D/proc/$PID/pagemap ibs=3D8 skip=3D$(($VADDR / $PAGESIZE)) count= =3D256 | hexdump -C 00000000 80 13 01 00 00 00 00 40 a0 13 01 00 00 00 00 40 |.......@..= .....@| ...........more swapped-out entries............ 000005e0 e0 2a 01 00 00 00 00 40 00 2b 01 00 00 00 00 40 |.*.....@.+= .....@| 000005f0 20 2b 01 00 00 00 00 40 40 2b 01 00 00 00 00 40 | +.....@@+= .....@| 00000600 72 6c 1d 00 00 00 80 a1 c1 34 12 00 00 00 80 a1 |rl.......4= ......| ...........more in-memory entries............ 000007f0 3c 21 18 00 00 00 80 a1 69 ec 17 00 00 00 80 a1 | Content-Transfer-Encoding: quoted-printable
On 14 Jul 2021, at 17:08, Peter Xu <peterx@redhat.com> wrote:

On Wed, Jul 14, 2021 at 03:24:26PM +0000, Tiberiu Georgescu= wrote:
static pagemap_entry_t pte_to_pagemap_= entry(struct pagemapread *pm,
struct vm_area_struct = *vma, unsigned long addr, pte_t pte)
{
u64 frame = =3D 0, flags =3D 0;
struct page= *page =3D NULL;

+ if (vma-&= gt;vm_flags & VM_SOFTDIRTY)
+ flags |=3D PM_SOFT_D= IRTY;
+
if (pte_pre= sent(pte)) {
if (pm->show_pfn) frame =3D pte_pfn(pte);
@@ -1374,13 +1387,22 @@ static pagemap_entry_t pte_to_pagemap_entry(struct = pagemapread *pm,
flags |=3D PM_SOFT_DIRTY;
if (pte_uffd_wp(pte))<= br class=3D""> flags |=3D PM_UFFD_WP;
- } else if= (is_swap_pte(pte)) {
+ } else if= (is_swap_pte(pte) || shmem_file(vma->vm_file)) {
swp_entry_t entry;
- if (pte_swp_soft_dir= ty(pte))
- flags |=3D PM_SOFT_DIRTY;
- if (pte_swp_uffd_wp(= pte))
- flags |=3D PM_UFFD_WP;
- entry =3D pte_to_swp= _entry(pte);
+ if (is_swap_pte(pte)= ) {
+ entry =3D pte_to_swp_entry(pte)= ;
+ if (pte_swp_soft_dirty(pte)) + flags |=3D PM_SOFT_DIRTY;
+ if (pte_swp_uffd_wp(pte))
+ flags |=3D PM_UFFD_WP;
+ } else {
+ void *xa_entry =3D get_xa_entry= _at_vma_addr(vma, addr);
+
+ if (xa_is_value(xa_entry))
+ entry =3D radix_to_swp_entry(xa_entry); + else
+ goto out;
+ }
if (pm->show_pfn) frame =3D swp_type(entry) |
(swp_offset(entry) << MAX_SWAPFILES_SHIFT);
@@ -1393,9 +1415,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pa= gemapread *pm,
flags |=3D PM_FILE; if (page &a= mp;& page_mapcount(page) =3D=3D 1)
flags |=3D PM_MMAP_EXC= LUSIVE;
- if (vma-&= gt;vm_flags & VM_SOFTDIRTY)
- flags |=3D PM_SOFT_D= IRTY;

IMHO moving this to the entry will only work for the initial iteration, how= ever
it won't really help anything, as soft-dirty should always be used in pair = with
clear_refs written with value "4" first otherwise all pages will = be marked
soft-dirty then the pagemap data is meaningless.

After the "write 4" op VM_SOFTDIRTY will be cleared and I expect = the test case
to see all zeros again even with the patch.

Indeed, the SOFT_DIRTY bit gets cleared and does not get set when we d= irty the
page and swap it out again. However, the pagemap entries are not compl= etely 
zeroed out. The patch mostly deals with adding the swap frame offset o= n the 
pagemap entries of swappable, non-syncable pages, even if they are MAP= _SHARED.

Example output post-patch, after writing 4 to clear_refs and dirtying = the pages:
        
$ dd= if=3D/proc/$PID/pagemap ibs=3D8 skip=3D$(($VADDR / $PAGESIZE)) count=3D256= | hexdump -C
    00000000  80 13 01 00 00 00 00 40  a0 13 01 00 00 00 00 40  |.......@.......@|
    ...........more swapped-out entries...........= .
&nbs= p;   000005e0  e0 2a 01 00 00 00 00 40  00 2b 01 00 00 00 00= 40  |.*.....@.+.....@|
&nbs= p;   000005f0  20 2b 01 00 00 00 00 40  40 2b 01= 00 00 00 00 40  | +.....@@+.....@|
&nbs= p;   00000600  72 6c 1d 00 00 00 80 a1  c1 34 12= 00 00 00 80 a1  |rl.......4......|
&nbs= p;   ...........more in-memory entries............
&nbs= p;   000007f0  3c 21 18 00 00 00 80 a1  69 ec 17= 00 00 00 80 a1  |<!......i.......|

You may find the pre-patch example output on the RFC cover letter, for= reference:

I think one way to fix this is to do something similar to u= ffd-wp: we leave a
marker in pte showing that this is soft-dirtied pte even if swapped out. However we don't have a mechanism for that yet in current linux, and the uffd-wp series is the first one trying to introduce something like that.

I am taking a look at your patch today. Hope it gets upstreamed soon, = so I can adapt
one of your mechanisms to keep track of the SOFT_DIRTY bit on the PTE = after swap.

Kind regards,
Tibi
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