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Date: Thu, 12 Dec 2019 12:33:41 +0100
From: Thomas Hellström (VMware)
<thomas_os@...pmail.org>
To: Steven Price <steven.price@....com>
Cc: Andy Lutomirski <luto@...nel.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
Catalin Marinas <catalin.marinas@....com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
James Morse <james.morse@....com>,
Jérôme Glisse <jglisse@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Will Deacon <will@...nel.org>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Mark Rutland <Mark.Rutland@....com>,
"Liang, Kan" <kan.liang@...ux.intel.com>,
Zong Li <zong.li@...ive.com>,
Andrew Morton <akpm@...ux-foundation.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v16 11/25] mm: pagewalk: Add p4d_entry() and pgd_entry()
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.
>
> 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@...ive.com>
>> Signed-off-by: Steven Price <steven.price@....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.
>
> The problem with the current patch is that the hmm pud_entry will
> traverse also pmds, so that will now be done twice.
>
> /Thomas
>
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