Re: [PATCH v5 7/9] mm/mremap: Move TLB flush outside page table lock

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:

> On 5/21/21 8:10 AM, Linus Torvalds wrote:
>> On Thu, May 20, 2021 at 6:57 AM Aneesh Kumar K.V
>> <aneesh.kumar@linux.ibm.com> wrote:
>>>
>>> Wondering whether this is correct considering we are holding mmap_sem in
>>> write mode in mremap.
>> 
>> Right. So *normally* the rule is to EITHER
>> 
>>   - hold the mmap_sem for writing
>> 
>> OR
>> 
>>   - hold the page table lock
>> 
>> and that the TLB flush needs to happen before you release that lock.
>> 
>> But as that commit message of commit eb66ae030829 ("mremap: properly
>> flush TLB before releasing the page") says, "mremap()" is a bit
>> special. It's special because mremap() didn't take ownership of the
>> page - it only moved it somewhere else. So now the page-out logic -
>> that relies on the page table lock - can free the page immediately
>> after we've released the page table lock.
>> 
>> So basically, in order to delay the TLB flush after releasing the page
>> table lock, it's not really sufficient to _just_ hold the mmap_sem for
>> writing. You also need to guarantee that the lifetime of the page
>> itself is held until after the TLB flush.
>> 
>> For normal operations like "munmap()", this happens naturally, because
>> we remove the page from the page table, and add it to the list of
>> pages to be freed after the TLB flush.
>> 
>> But mremap never did that "remove the page and add it to a list to be
>> free'd later". Instead, it just moved the page somewhere else. And
>> thus there is no guarantee that the page that got moved will continue
>> to exist until a TLB flush is done.
>> 
>> So mremap does need to flush the TLB before releasing the page table
>> lock, because that's the lifetime boundary for the page that got
>> moved.
>
> How will we avoid that happening with 
> c49dd340180260c6239e453263a9a244da9a7c85 / 
> 2c91bd4a4e2e530582d6fd643ea7b86b27907151 . The commit improves mremap 
> performance by moving level3/level2 page table entries. When doing so we 
> are not holding level 4 ptl lock (pte_lock()). But rather we are holding 
> pmd_lock or pud_lock(). So if we move pages around without holding the 
> pte lock, won't the above issue happen even if we do a tlb flush with 
> holding pmd lock/pud lock?

This should help? ie, we flush tlb before we move pagetables to the new
address? 

modified   mm/mremap.c
@@ -277,11 +277,14 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
 	/* Clear the pmd */
 	pmd = *old_pmd;
 	pmd_clear(old_pmd);
-
+	/*
+	 * flush the TLB before we move the page table entries.
+	 * TLB flush includes necessary barriers.
+	 */
+	flush_pte_tlb_pwc_range(vma, old_addr, old_addr + PMD_SIZE);
 	VM_BUG_ON(!pmd_none(*new_pmd));
 	pmd_populate(mm, new_pmd, pmd_pgtable(pmd));
 
-	flush_pte_tlb_pwc_range(vma, old_addr, old_addr + PMD_SIZE);
 	if (new_ptl != old_ptl)
 		spin_unlock(new_ptl);
 	spin_unlock(old_ptl);


-aneesh