Messages in this thread | | | Subject | Re: [PATCH v16 11/25] mm: pagewalk: Add p4d_entry() and pgd_entry() | From | Thomas Hellström (VMware) <> | Date | Thu, 12 Dec 2019 15:04:29 +0100 |
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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.
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 */ }
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
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