linux-kernel - Re: [PATCH] Initialise mmu_notifier

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Date:   Wed, 2 Jan 2019 20:18:33 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jerome Glisse <jglisse@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christian König <christian.koenig@....com>,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] Initialise mmu_notifier_range correctly

On Wed, Jan 02, 2019 at 07:32:08PM -0800, John Hubbard wrote:
> Having the range struct declared in separate places from the mmu_notifier_range_init()
> calls is not great. But I'm not sure I see a way to make it significantly cleaner, given
> that __follow_pte_pmd uses the range pointer as a way to decide to issue the mmn calls.

Yeah, I don't think there's anything we can do.  But I started reviewing
the comments, and they don't make sense together:

                /*
                 * Note because we provide range to follow_pte_pmd it will
                 * call mmu_notifier_invalidate_range_start() on our behalf
                 * before taking any lock.
                 */
                if (follow_pte_pmd(vma->vm_mm, address, &range,
                                   &ptep, &pmdp, &ptl))
                        continue;

                /*
                 * No need to call mmu_notifier_invalidate_range() as we are
                 * downgrading page table protection not changing it to point
                 * to a new page.
                 *
                 * See Documentation/vm/mmu_notifier.rst
                 */

So if we don't call mmu_notifier_invalidate_range, why are we calling
mmu_notifier_invalidate_range_start and mmu_notifier_invalidate_range_end?
ie, why not this ...

diff --git a/fs/dax.c b/fs/dax.c
index 6959837cc465..905340149924 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -777,7 +777,6 @@ static void dax_entry_mkclean(struct address_space *mapping, pgoff_t index,
 
 	i_mmap_lock_read(mapping);
 	vma_interval_tree_foreach(vma, &mapping->i_mmap, index, index) {
-		struct mmu_notifier_range range;
 		unsigned long address;
 
 		cond_resched();
@@ -787,12 +786,7 @@ static void dax_entry_mkclean(struct address_space *mapping, pgoff_t index,
 
 		address = pgoff_address(index, vma);
 
-		/*
-		 * Note because we provide start/end to follow_pte_pmd it will
-		 * call mmu_notifier_invalidate_range_start() on our behalf
-		 * before taking any lock.
-		 */
-		if (follow_pte_pmd(vma->vm_mm, address, &range,
+		if (follow_pte_pmd(vma->vm_mm, address, NULL,
 				   &ptep, &pmdp, &ptl))
 			continue;
 
@@ -834,8 +828,6 @@ static void dax_entry_mkclean(struct address_space *mapping, pgoff_t index,
 unlock_pte:
 			pte_unmap_unlock(ptep, ptl);
 		}
-
-		mmu_notifier_invalidate_range_end(&range);
 	}
 	i_mmap_unlock_read(mapping);
 }