From: Steven Price <steven.price@arm.com>
To: "Thomas Hellström (VMware)" <thomas_os@shipmail.org>
Cc: "Mark Rutland" <Mark.Rutland@arm.com>,
x86@kernel.org, "Zong Li" <zong.li@sifive.com>,
"Arnd Bergmann" <arnd@arndb.de>,
"Ard Biesheuvel" <ard.biesheuvel@linaro.org>,
"Peter Zijlstra" <peterz@infradead.org>,
"Catalin Marinas" <catalin.marinas@arm.com>,
"Dave Hansen" <dave.hansen@linux.intel.com>,
linux-kernel@vger.kernel.org,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"Jérôme Glisse" <jglisse@redhat.com>,
"Ingo Molnar" <mingo@redhat.com>,
"Borislav Petkov" <bp@alien8.de>,
"Andy Lutomirski" <luto@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
"James Morse" <james.morse@arm.com>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Will Deacon" <will@kernel.org>,
"Andrew Morton" <akpm@linux-foundation.org>,
linux-arm-kernel@lists.infradead.org, "Liang,
Kan" <kan.liang@linux.intel.com>
Subject: Re: [PATCH v16 11/25] mm: pagewalk: Add p4d_entry() and pgd_entry()
Date: Thu, 12 Dec 2019 15:18:33 +0000 [thread overview]
Message-ID: <2cef327b-a16b-f76d-4c3f-bd894332c736@arm.com> (raw)
In-Reply-To: <16b2ecbc-316a-33f8-ace2-e54cd8001b24@shipmail.org>
On 12/12/2019 14:04, Thomas Hellström (VMware) wrote:
> On 12/12/19 2:15 PM, Steven Price wrote:
>> On 12/12/2019 11:33, Thomas Hellström (VMware) wrote:
>>> On 12/12/19 12:23 PM, Thomas Hellström (VMware) wrote:
>>>> On 12/6/19 2:53 PM, Steven Price wrote:
>>>>> pgd_entry() and pud_entry() were removed by commit 0b1fbfe50006c410
>>>>> ("mm/pagewalk: remove pgd_entry() and pud_entry()") because there were
>>>>> no users. We're about to add users so reintroduce them, along with
>>>>> p4d_entry() as we now have 5 levels of tables.
>>>>>
>>>>> Note that commit a00cc7d9dd93d66a ("mm, x86: add support for
>>>>> PUD-sized transparent hugepages") already re-added pud_entry() but with
>>>>> different semantics to the other callbacks. Since there have never
>>>>> been upstream users of this, revert the semantics back to match the
>>>>> other callbacks. This means pud_entry() is called for all entries, not
>>>>> just transparent huge pages.
>>
>> When I wrote that there were no upstream users, which sadly shows how
>> long ago that was :(
>>
>>>> Actually, there are two users of pud_entry(), in hmm.c and since 5.5rc1 also mapping_dirty_helpers.c. The latter one is unproblematic and requires no attention but the one in hmm.c is probably largely untested, and seems to assume it was called outside of the spinlock.
>>>>
>>>> The problem with the current patch is that the hmm pud_entry will traverse also pmds, so that will be done twice now.
>>>>
>>>> In another thread we were discussing a means of rerunning the level (in case of a race), or continuing after a level, based on the return value after the callback. The change was fairly invasive,
>>>>
>>> Hmm. Forgot to remove the above text that appears twice. :(. The correct one is inline below.
>>>
>>>>
>>>>> Tested-by: Zong Li <zong.li@sifive.com>
>>>>> Signed-off-by: Steven Price <steven.price@arm.com>
>>>>> ---
>>>>> include/linux/pagewalk.h | 19 +++++++++++++------
>>>>> mm/pagewalk.c | 27 ++++++++++++++++-----------
>>>>> 2 files changed, 29 insertions(+), 17 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
>>>>> index 6ec82e92c87f..06790f23957f 100644
>>>>> --- a/include/linux/pagewalk.h
>>>>> +++ b/include/linux/pagewalk.h
>>>>> @@ -8,15 +8,15 @@ struct mm_walk;
>>>>> /**
>>>>> * mm_walk_ops - callbacks for walk_page_range
>>>>> - * @pud_entry: if set, called for each non-empty PUD (2nd-level) entry
>>>>> - * this handler should only handle pud_trans_huge() puds.
>>>>> - * the pmd_entry or pte_entry callbacks will be used for
>>>>> - * regular PUDs.
>>>>> - * @pmd_entry: if set, called for each non-empty PMD (3rd-level) entry
>>>>> + * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
>>>>> + * @p4d_entry: if set, called for each non-empty P4D entry
>>>>> + * @pud_entry: if set, called for each non-empty PUD entry
>>>>> + * @pmd_entry: if set, called for each non-empty PMD entry
>>>>> * this handler is required to be able to handle
>>>>> * pmd_trans_huge() pmds. They may simply choose to
>>>>> * split_huge_page() instead of handling it explicitly.
>>>>> - * @pte_entry: if set, called for each non-empty PTE (4th-level) entry
>>>>> + * @pte_entry: if set, called for each non-empty PTE (lowest-level)
>>>>> + * entry
>>>>> * @pte_hole: if set, called for each hole at all levels
>>>>> * @hugetlb_entry: if set, called for each hugetlb entry
>>>>> * @test_walk: caller specific callback function to determine whether
>>>>> @@ -27,8 +27,15 @@ struct mm_walk;
>>>>> * @pre_vma: if set, called before starting walk on a non-null vma.
>>>>> * @post_vma: if set, called after a walk on a non-null vma, provided
>>>>> * that @pre_vma and the vma walk succeeded.
>>>>> + *
>>>>> + * p?d_entry callbacks are called even if those levels are folded on a
>>>>> + * particular architecture/configuration.
>>>>> */
>>>>> struct mm_walk_ops {
>>>>> + int (*pgd_entry)(pgd_t *pgd, unsigned long addr,
>>>>> + unsigned long next, struct mm_walk *walk);
>>>>> + int (*p4d_entry)(p4d_t *p4d, unsigned long addr,
>>>>> + unsigned long next, struct mm_walk *walk);
>>>>> int (*pud_entry)(pud_t *pud, unsigned long addr,
>>>>> unsigned long next, struct mm_walk *walk);
>>>>> int (*pmd_entry)(pmd_t *pmd, unsigned long addr,
>>>>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>>>>> index ea0b9e606ad1..c089786e7a7f 100644
>>>>> --- a/mm/pagewalk.c
>>>>> +++ b/mm/pagewalk.c
>>>>> @@ -94,15 +94,9 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>>>>> }
>>>>> if (ops->pud_entry) {
>>>>> - spinlock_t *ptl = pud_trans_huge_lock(pud, walk->vma);
>>>>> -
>>>>> - if (ptl) {
>>>>> - err = ops->pud_entry(pud, addr, next, walk);
>>>>> - spin_unlock(ptl);
>>>>> - if (err)
>>>>> - break;
>>>>> - continue;
>>>>> - }
>>>>> + err = ops->pud_entry(pud, addr, next, walk);
>>>>> + if (err)
>>>>> + break;
>>>>
>>>> Actually, there are two current users of pud_entry(), in hmm.c and since 5.5rc1 also mapping_dirty_helpers.c. The latter one is unproblematic and requires no attention but the one in hmm.c is probably largely untested, and seems to assume it was called outside of the spinlock.
>>
>> Thanks for pointing that out, I guess the simplest fix would be to
>> squash in something like the below which should restore the old
>> behaviour for hmm.c without affecting others.
>>
>> Steve
>
> I'm not fully sure that the old behaviour is the correct one, but definitely hmm's pud_entry needs some fixing.
> I'm more concerned with the pagewalk code. With your patch it actually splits all huge puds present in the page-table
> on each page walk which is not what we want.
Good catch - yes that's certainly not ideal.
> One idea would be to add a new member to struct_mm_walk:
>
> enum page_walk_ret_action {
> ACTION_SUBTREE = 0,
> ACTION_CONTINUE = 1,
> ACTION_AGAIN = 2 /* Only for levels that thave p?d_unstable */
> };
>
> struct mm_walk {
> ...
> enum page_walk_ret_action action; /* or perhaps as an enum */
> };
>
>
> if (ops->pud_entry) {
> walk->action = ACTION_SUBTREE;
> ...
> ...
> ...
> if (walk->action == ACTION_AGAIN) /* Callback tried to split huge entry, but failed */
> goto again;
> else if (walk->action == ACTION_CONTINUE) /* Done with this subtree. Probably huge entry handled. */
> continue;
> /* ACTION_SUBTREE falls through */
> }
I'll have a go at implementing the above - this might also allow removing the test_p?d() callbacks as they can simply return ACTION_CONTINUE.
Steve
> we discussed something similar before on linux-mm, but the idea then was to redefine
> the positive return value of the callback to the action, but that meant changing those existing callbacks that relied on
> a positive return value. The above would be helpful also for pmd_entry.
>
> /Thomas
>
>
>
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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next prev parent reply other threads:[~2019-12-12 15:18 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-06 13:52 [PATCH v16 00/25] Generic page walk and ptdump Steven Price
2019-12-06 13:52 ` [PATCH v16 01/25] mm: Add generic p?d_leaf() macros Steven Price
2019-12-06 13:52 ` [PATCH v16 02/25] arc: mm: Add p?d_leaf() definitions Steven Price
2019-12-06 13:52 ` [PATCH v16 03/25] arm: " Steven Price
2019-12-06 13:52 ` [PATCH v16 04/25] arm64: " Steven Price
2019-12-06 13:52 ` [PATCH v16 05/25] mips: " Steven Price
2019-12-06 13:52 ` [PATCH v16 06/25] powerpc: " Steven Price
2019-12-09 11:08 ` Michael Ellerman
2019-12-09 13:06 ` Steven Price
2019-12-06 13:52 ` [PATCH v16 07/25] riscv: " Steven Price
2019-12-06 13:52 ` [PATCH v16 08/25] s390: " Steven Price
2019-12-06 13:53 ` [PATCH v16 09/25] sparc: " Steven Price
2019-12-06 13:53 ` [PATCH v16 10/25] x86: " Steven Price
2019-12-06 13:53 ` [PATCH v16 11/25] mm: pagewalk: Add p4d_entry() and pgd_entry() Steven Price
2019-12-12 11:23 ` Thomas Hellström (VMware)
2019-12-12 11:33 ` Thomas Hellström (VMware)
2019-12-12 13:15 ` Steven Price
2019-12-12 14:04 ` Thomas Hellström (VMware)
2019-12-12 15:18 ` Steven Price [this message]
2019-12-06 13:53 ` [PATCH v16 12/25] mm: pagewalk: Allow walking without vma Steven Price
2019-12-06 13:53 ` [PATCH v16 13/25] mm: pagewalk: Don't lock PTEs for walk_page_range_novma() Steven Price
2019-12-10 11:23 ` kbuild test robot
2019-12-11 15:54 ` Steven Price
2019-12-11 17:12 ` Luc Van Oostenryck
2019-12-11 17:19 ` Qian Cai
2019-12-06 13:53 ` [PATCH v16 14/25] mm: pagewalk: fix termination condition in walk_pte_range() Steven Price
2019-12-06 13:53 ` [PATCH v16 15/25] mm: pagewalk: Add test_p?d callbacks Steven Price
2019-12-06 13:53 ` [PATCH v16 16/25] mm: pagewalk: Add 'depth' parameter to pte_hole Steven Price
2019-12-06 13:53 ` [PATCH v16 17/25] x86: mm: Point to struct seq_file from struct pg_state Steven Price
2019-12-06 13:53 ` [PATCH v16 18/25] x86: mm+efi: Convert ptdump_walk_pgd_level() to take a mm_struct Steven Price
2019-12-06 13:53 ` [PATCH v16 19/25] x86: mm: Convert ptdump_walk_pgd_level_debugfs() to take an mm_struct Steven Price
2019-12-06 13:53 ` [PATCH v16 20/25] x86: mm: Convert ptdump_walk_pgd_level_core() " Steven Price
2019-12-06 13:53 ` [PATCH v16 21/25] mm: Add generic ptdump Steven Price
2019-12-06 13:53 ` [PATCH v16 22/25] x86: mm: Convert dump_pagetables to use walk_page_range Steven Price
2019-12-06 13:53 ` [PATCH v16 23/25] arm64: mm: Convert mm/dump.c to use walk_page_range() Steven Price
2019-12-06 13:53 ` [PATCH v16 24/25] arm64: mm: Display non-present entries in ptdump Steven Price
2019-12-06 13:53 ` [PATCH v16 25/25] mm: ptdump: Reduce level numbers by 1 in note_page() Steven Price
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