Re: [PATCH 06/10] mm/mmap/vma_merge: set mid to NULL if not applicable

[Vlastimil Babka <vbabka@suse.cz>]

On 3/15/23 22:34, Lorenzo Stoakes wrote:
> On Thu, Mar 09, 2023 at 12:12:54PM +0100, Vlastimil Babka wrote:
>> There are several places where we test if 'mid' is really the area NNNN
>> in the diagram and the tests have two variants and are non-obvious to
>> follow.  Instead, set 'mid' to NULL up-front if it's not the NNNN area,
>> and simplify the tests.
>>
>> Also update the description in comment accordingly.
>>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>>  mm/mmap.c | 21 +++++++++++++--------
>>  1 file changed, 13 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index be60b344e4b1..3396c9b13f1c 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -848,10 +848,11 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
>>   *
>>   * The following mprotect cases have to be considered, where AAAA is
>>   * the area passed down from mprotect_fixup, never extending beyond one
>> - * vma, PPPPPP is the prev vma specified, and NNNNNN the next vma after:
>> + * vma, PPPPPP is the prev vma specified, NNNN is a vma that overlaps
>> + * the area AAAA and XXXXXX the next vma after AAAA:
> 
> I think this is worded in a bit of a confusing way + can be read as 'NNNN is a
> vma that overlaps the area AAAA and XXXXXX' whereas you mean to say 'NNNN is a
> VMA that overlaps the area AAAA, and XXXXXX is the next vma after AAAA'.
> 
> This therefore might be better worded as:-
> 
> 'PPPP is the previous VMA, NNNN is a VMA which overlaps AAAA and XXXX is the
> next VMA after AAAA.'
> 
> Also - nit, but there's also inconsistency here between the number of letters in
> each block, e.g. 6 P's 4 N's 4 A's and 6 X's.

OK, I fixed that up (-fix patch below), thanks. Note that it's not just
"overlaps" for NNNN, it also has to align at the start of AAAA, so I made
that explicit in the comment. It also means PPPP no longer "overlaps" by
this definition in case 4.

> 'N' and 'X' are starting to be horrifically misleading here imo, I feel as if
> 'N' moving to 'O' (for overlapping) and 'X' to 'N' would make a big difference
> here.
 
I'll leave that possibility for a future patch as that's easier to done at
once after all those incremental changes here. But again note how
"overlapping" is not completely accurate word due to the start alignemnt.

>>   *
>>   *     AAAA             AAAA                   AAAA
>> - *    PPPPPPNNNNNN    PPPPPPXXXXXX       PPPPPPNNNNNN
>> + *    PPPPPPXXXXXX    PPPPPPXXXXXX       PPPPPPNNNNNN
>>   *    cannot merge    might become       might become
>>   *                    PPXXXXXXXXXX       PPPPPPPPPPNN
>>   *    mmap, brk or    case 4 below       case 5 below
>> @@ -879,9 +880,10 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
>>   *
>>   * In the code below:
>>   * PPPP is represented by *prev
>> - * NNNN is represented by *mid (and possibly equal to *next)
>> - * XXXX is represented by *next or not represented at all.
>> - * AAAA is not represented - it will be merged or the function will return NULL
>> + * NNNN is represented by *mid or not represented at all (NULL)
>> + * XXXX is represented by *next or not represented at all (NULL)
>> + * AAAA is not represented - it will be merged and the vma containing the
>> + *      area is returned, or the function will return NULL
>>   */
>>  struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>>  			struct vm_area_struct *prev, unsigned long addr,
>> @@ -918,6 +920,9 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>>  	else
>>  		next = mid;
>>
>> +	if (mid && end <= mid->vm_start)
>> +		mid = NULL;
>> +
> 
> Might be worth putting a comment with the cases where this will happen, 1 - 4
> right? And also something like 'does AAAA overlap with mid?'

Added to the -fix patch below, with slightly different comment.

> And I really think renaming this to 'overlapping' or 'overlaps' or similar would
> make a big readability difference.

Yeah but it's quite long word and again not completely self explanatory.

> However we do have the thorny issue of case 4 where A overlaps P... But probably
> the fact that we treat this as a separate VMA from prev is enough to make it
> clear it being called 'overlaps' means 'separate from prev, also overlaps' so I
> think that's fine.
> 
> Adding this actually makes me think twice about the previous 'natural order'
> patch, because the intuition which that promotes is:-
> 
> mid = VMA after prev
> next = VMA after mid
> 
> [ prev ] [ mid ] [ next ]
> 
> But in reality that else branch means that next could be be equal to mid and
> now if there isn't overlap we rename mid to next effectively, e.g.:-
> 
> mid = VMA after prev
> next = mid
> delete mid
> 
> Which feels like the 'natural' intuition is suddenly broken. Maybe this needs
> reworking to be super explicit about this? Such as:-
> 
> struct vm_area_struct tmp;
> 
> ...
> 
> /* If there is a previous VMA, find the next, otherwise find the first. */
> tmp = find_vma(mm, prev ? prev->vm_end : 0);
> 
> /*
>  * If the address range overlaps with the input range (which can cover only a
>  * single VMA at most), then we are only interested in next if we span right up
>  * to its end.
>  *
>  * Otherwise we are simply left with prev and next.
>  */
> overlaps = tmp && end > tmp->vm_start ? tmp : NULL;
> if (overlaps)
> 	next = overlaps->vm_end == end ? overlaps->vm_next : NULL;
> else
> 	next = tmp;
> 
> Of course I haven't read the rest of the patches in this series so you may
> address aspects of this already :)

So as said above feel free to propose further followup in that direction.
You're right that in case 5 we should end up with next == NULL, in order to
be completely accurate. If we made sure next is only non-NULL if "end ==
next->vm_start" upfront, we could leave out that test later in "/* Can we
merge the successor? */".

>>  	/* verify some invariant that must be enforced by the caller */
>>  	VM_WARN_ON(prev && addr <= prev->vm_start);
>>  	VM_WARN_ON(mid && end > mid->vm_end);
>> @@ -952,7 +957,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>>  		remove = next;				/* case 1 */
>>  		vma_end = next->vm_end;
>>  		err = dup_anon_vma(prev, next);
>> -		if (mid != next) {			/* case 6 */
>> +		if (mid) {				/* case 6 */
>>  			remove = mid;
>>  			remove2 = next;
>>  			if (!next->anon_vma)
>> @@ -960,7 +965,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>>  		}
>>  	} else if (merge_prev) {
>>  		err = 0;				/* case 2 */
>> -		if (mid && end > mid->vm_start) {
>> +		if (mid) {
>>  			err = dup_anon_vma(prev, mid);
>>  			if (end == mid->vm_end) {	/* case 7 */
>>  				remove = mid;
>> @@ -982,7 +987,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>>  			vma_end = next->vm_end;
>>  			vma_pgoff = next->vm_pgoff;
>>  			err = 0;
>> -			if (mid != next) {		/* case 8 */
>> +			if (mid) {			/* case 8 */
>>  				vma_pgoff = mid->vm_pgoff;
>>  				remove = mid;
>>  				err = dup_anon_vma(next, mid);
>> --
>> 2.39.2
>>
> 
> Other than the nitty comment notes and the conceptual discussion, this LGTM so:-
> 
> Reviewed-By: Lorenzo Stoakes <lstoakes@gmail.com>

Thanks! Here's the -fix patch:

----8<----
From 1016590e31f0173070daffd905c3396607a68b4b Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Thu, 16 Mar 2023 10:56:04 +0100
Subject: [PATCH] mm/mmap/vma_merge: set mid to NULL if not applicable-fix

Adjust/add comments as suggested by Lorenzo.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/mmap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 3396c9b13f1c..cd0b0d1f4aeb 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -848,8 +848,9 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
  *
  * The following mprotect cases have to be considered, where AAAA is
  * the area passed down from mprotect_fixup, never extending beyond one
- * vma, PPPPPP is the prev vma specified, NNNN is a vma that overlaps
- * the area AAAA and XXXXXX the next vma after AAAA:
+ * vma, PPPP is the previous vma, NNNN is a vma that starts at the same
+ * address as AAAA and is of the same or larger span, and XXXX the next
+ * vma after AAAA:
  *
  *     AAAA             AAAA                   AAAA
  *    PPPPPPXXXXXX    PPPPPPXXXXXX       PPPPPPNNNNNN
@@ -920,6 +921,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 	else
 		next = mid;
 
+	/* In cases 1 - 4 there's no NNNN vma */
 	if (mid && end <= mid->vm_start)
 		mid = NULL;
 
-- 
2.39.2