| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <25734c2a-d79a-4a16-8ec4-0b6e1b7a59b5@amd.com>
Date: Mon, 14 Jul 2025 19:13:00 +0530
From: Shivank Garg <shivankg@....com>
To: Luiz Capitulino <luizcap@...hat.com>, david@...hat.com,
willy@...radead.org, akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, sj@...nel.org
Subject: Re: [PATCH v2 2/4] mm/util: introduce snapshot_page()
On 7/14/2025 6:46 PM, Luiz Capitulino wrote:
> On 2025-07-08 12:59, Luiz Capitulino wrote:
>> On 2025-07-08 01:49, Shivank Garg wrote:
>>>
>>>
>>> On 7/8/2025 12:20 AM, Luiz Capitulino wrote:
>>>> This commit refactors __dump_page() into snapshot_page().
>>>>
>>>> snapshot_page() tries to take a faithful snapshot of a page and its
>>>> folio representation. The snapshot is returned in the struct
>>>> page_snapshot parameter along with additional flags that are best
>>>> retrieved at snapshot creation time to reduce race windows.
>>>>
>>>> This function is intended to be used by callers that need a stable
>>>> representation of a struct page and struct folio so that pointers
>>>> or page information doesn't change while working on a page.
>>>>
>>>> The idea and original implementation of snapshot_page() comes from
>>>> Matthew Wilcox with suggestions for improvements from David Hildenbrand.
>>>> All bugs and misconceptions are mine.
>>>>
>>>> Signed-off-by: Luiz Capitulino <luizcap@...hat.com>
>>>> ---
>>>> include/linux/mm.h | 19 ++++++++++++
>>>> mm/debug.c | 42 +++----------------------
>>>> mm/util.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++
>>>> 3 files changed, 100 insertions(+), 38 deletions(-)
>>>>
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index 0ef2ba0c667a..090968c6eebb 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -4184,4 +4184,23 @@ static inline bool page_pool_page_is_pp(struct page *page)
>>>> }
>>>> #endif
>>>> +#define PAGE_SNAPSHOT_FAITHFUL (1 << 0)
>>>> +#define PAGE_SNAPSHOT_PG_FREE (1 << 1)
>>>> +#define PAGE_SNAPSHOT_PG_IDLE (1 << 2)
>>>> +
>>>> +struct page_snapshot {
>>>> + struct folio folio_snapshot;
>>>> + struct page page_snapshot;
>>>> + unsigned long pfn;
>>>> + unsigned long idx;
>>>> + unsigned long flags;
>>>> +};
>>>> +
>>>> +static inline bool snapshot_page_is_faithful(const struct page_snapshot *ps)
>>>> +{
>>>> + return ps->flags & PAGE_SNAPSHOT_FAITHFUL;
>>>> +}
>>>> +
>>>> +void snapshot_page(struct page_snapshot *ps, const struct page *page);
>>>> +
>>>> #endif /* _LINUX_MM_H */
>>>> diff --git a/mm/debug.c b/mm/debug.c
>>>> index 907382257062..7349330ea506 100644
>>>> --- a/mm/debug.c
>>>> +++ b/mm/debug.c
>>>> @@ -129,47 +129,13 @@ static void __dump_folio(struct folio *folio, struct page *page,
>>>> static void __dump_page(const struct page *page)
>>>> {
>>>> - struct folio *foliop, folio;
>>>> - struct page precise;
>>>> - unsigned long head;
>>>> - unsigned long pfn = page_to_pfn(page);
>>>> - unsigned long idx, nr_pages = 1;
>>>> - int loops = 5;
>>>> -
>>>> -again:
>>>> - memcpy(&precise, page, sizeof(*page));
>>>> - head = precise.compound_head;
>>>> - if ((head & 1) == 0) {
>>>> - foliop = (struct folio *)&precise;
>>>> - idx = 0;
>>>> - if (!folio_test_large(foliop))
>>>> - goto dump;
>>>> - foliop = (struct folio *)page;
>>>> - } else {
>>>> - foliop = (struct folio *)(head - 1);
>>>> - idx = folio_page_idx(foliop, page);
>>>> - }
>>>> + struct page_snapshot ps;
>>>> - if (idx < MAX_FOLIO_NR_PAGES) {
>>>> - memcpy(&folio, foliop, 2 * sizeof(struct page));
>>>> - nr_pages = folio_nr_pages(&folio);
>>>> - if (nr_pages > 1)
>>>> - memcpy(&folio.__page_2, &foliop->__page_2,
>>>> - sizeof(struct page));
>>>> - foliop = &folio;
>>>> - }
>>>> -
>>>> - if (idx > nr_pages) {
>>>> - if (loops-- > 0)
>>>> - goto again;
>>>> + snapshot_page(&ps, page);
>>>> + if (!snapshot_page_is_faithful(&ps))
>>>> pr_warn("page does not match folio\n");
>>>> - precise.compound_head &= ~1UL;
>>>> - foliop = (struct folio *)&precise;
>>>> - idx = 0;
>>>> - }
>>>> -dump:
>>>> - __dump_folio(foliop, &precise, pfn, idx);
>>>> + __dump_folio(&ps.folio_snapshot, &ps.page_snapshot, ps.pfn, ps.idx);
>>>> }
>>>> void dump_page(const struct page *page, const char *reason)
>>>> diff --git a/mm/util.c b/mm/util.c
>>>> index 0b270c43d7d1..c38d213be83f 100644
>>>> --- a/mm/util.c
>>>> +++ b/mm/util.c
>>>> @@ -25,6 +25,7 @@
>>>> #include <linux/sizes.h>
>>>> #include <linux/compat.h>
>>>> #include <linux/fsnotify.h>
>>>> +#include <linux/page_idle.h>
>>>> #include <linux/uaccess.h>
>>>> @@ -1171,3 +1172,79 @@ int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
>>>> return 0;
>>>> }
>>>> EXPORT_SYMBOL(compat_vma_mmap_prepare);
>>>> +
>>>> +static void set_flags(struct page_snapshot *ps, const struct folio *folio,
>>>> + const struct page *page)
>>>> +{
>>>> + /*
>>>> + * Caveats on high order pages: PG_buddy and PG_slab will only be set
>>>> + * on the head page.
>>>> + */
>>>> + if (PageBuddy(page))
>>>> + ps->flags |= PAGE_SNAPSHOT_PG_FREE;
>>>> + else if (page_count(page) == 0 && is_free_buddy_page(page))
>>>> + ps->flags |= PAGE_SNAPSHOT_PG_FREE;
>>>> +
>>>> + if (folio_test_idle(folio))
>>>> + ps->flags |= PAGE_SNAPSHOT_PG_IDLE;
>>>> +}
>>>> +
>>>> +/*
>>>> + * Create a snapshot of a page and store its struct page and struct folio
>>>> + * representations in a struct page_snapshot.
>>>> + *
>>>> + * @ps: struct page_snapshot to store the page snapshot
>>>> + * @page: the page we want to snapshot
>>>> + *
>>>> + * Note that creating a faithful snapshot of a page may fail if the page
>>>> + * compound keeps changing (eg. due to folio split). In this case we set
>>>> + * ps->faithful to false and the snapshot will assume that @page refers
>>>> + * to a single page.
>>>> + */
>>>> +void snapshot_page(struct page_snapshot *ps, const struct page *page)
>>>> +{
>>>> + unsigned long head, nr_pages = 1;
>>>> + struct folio *foliop, folio;
>>>> + int loops = 5;
>>>> +
>>>> + ps->pfn = page_to_pfn(page);
>>>> + ps->flags = PAGE_SNAPSHOT_FAITHFUL;
>>>> +
>>>> +again:
>>>> + memcpy(&ps->page_snapshot, page, sizeof(*page));
>>>> + head = ps->page_snapshot.compound_head;
>>>> + if ((head & 1) == 0) {
>>>> + foliop = (struct folio *)&ps->page_snapshot;
>>>> + ps->idx = 0;
>>>> + if (!folio_test_large(foliop)) {
>>>> + set_flags(ps, page_folio(page), page);
>>>> + goto out;
>>>> + }
>>>> + foliop = (struct folio *)page;
>>>> + } else {
>>>> + foliop = (struct folio *)(page->compound_head - 1);
>>> ^^^^
>>> should we use ps->page_snapshot here?
>>> IIUC, page may change due to race.
>>
>> You're right the race exists, but we can't make foliop point to the
>> snapshot because we do pointer arithmetic and other operations that need
>> the real page memory address to work properly.
>
> While working on v3 I realized it should be fine (and correct) to use
> the head pointer from ps->page_snapshot like the original __dump_page()
> does (assuming this is what you meant for me to change). So, I changed
> this for v3.
>
Yes, that's what I meant.
Thanks for updating it.
>>
>>>> + ps->idx = folio_page_idx(foliop, page);
>>>> + }
>>>> +
>>>> + if (ps->idx < MAX_FOLIO_NR_PAGES) {
>>>> + memcpy(&folio, foliop, 2 * sizeof(struct page));
>>>> + nr_pages = folio_nr_pages(&folio);
>>>> + if (nr_pages > 1)
>>>> + memcpy(&folio.__page_2, &foliop->__page_2,
>>>> + sizeof(struct page));
>>>> + set_flags(ps, foliop, page);
>>>> + foliop = &folio;
>>>> + }
>>>> +
>>>> + if (ps->idx > nr_pages) {
>>>> + if (loops-- > 0)
>>>> + goto again;
>>>> + ps->page_snapshot.compound_head &= ~1UL;
>>>
>>> Should we use clear_compound_head() for clarity?
>>
>> Yes, we can do that.
>>
>> Andrew, since you applied this series already, can you squash the change
>> below into this patch? If not, I can send v3.
>>
>> diff --git a/mm/util.c b/mm/util.c
>> index c38d213be83f..ce4200529772 100644
>> --- a/mm/util.c
>> +++ b/mm/util.c
>> @@ -1239,7 +1239,7 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
>> if (ps->idx > nr_pages) {
>> if (loops-- > 0)
>> goto again;
>> - ps->page_snapshot.compound_head &= ~1UL;
>> + clear_compound_head(&ps->page_snapshot);
>> foliop = (struct folio *)&ps->page_snapshot;
>> ps->flags = 0;
>> ps->idx = 0;
>
Powered by blists - more mailing lists