Re: [Xen-devel] [PATCH] mm/page_alloc: correct first_dirty calculations during block merging

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.07.18 at 16:49, <sergey.dyasli@xxxxxxxxxx> wrote:
> Currently it's possible to hit an assertion in alloc_heap_pages():
> 
> Assertion 'first_dirty != INVALID_DIRTY_IDX || !(pg[i].count_info & 
> PGC_need_scrub)' failed at page_alloc.c:988
> 
> This can happen because a piece of logic to calculate first_dirty
> during block merging in free_heap_pages() is missing for the following
> scenario:
> 
> 1. Current block's first_dirty equals to INVALID_DIRTY_IDX
> 2. Successor block is free but its first_dirty != INVALID_DIRTY_IDX
> 3. The successor is merged into current block
> 4. Current block's first_dirty still equals to INVALID_DIRTY_IDX
> 
> This will trigger the assertion during allocation of such block in
> alloc_heap_pages() because there will be pages with PGC_need_scrub
> bit set despite the claim of first_dirty that the block is scrubbed.
> 
> Add the missing piece of logic and slightly update the comment for
> the predecessor case to better capture the code's intent.
> 
> Fixes 1a37f33ea613 ("mm: Place unscrubbed pages at the end of pagelist")
> 
> Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> ---
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> ---
>  xen/common/page_alloc.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
> index 20ee1e4897..aa911f2dc5 100644
> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -1426,7 +1426,7 @@ static void free_heap_pages(
>  
>              page_list_del(predecessor, &heap(node, zone, order));
>  
> -            /* Keep predecessor's first_dirty if it is already set. */
> +            /* Keep block's first_dirty if the predecessor doesn't have one 
> */
>              if ( predecessor->u.free.first_dirty == INVALID_DIRTY_IDX &&
>                   pg->u.free.first_dirty != INVALID_DIRTY_IDX )
>                  predecessor->u.free.first_dirty = (1U << order) +

How about "Convert pg's first_dirty if predecessor doesn't already have
one"? "Keep" isn't describing well enough what's being done here imo.
Also, despite both styles being okay, I'd prefer to retain the full stop.

> @@ -1447,6 +1447,12 @@ static void free_heap_pages(
>  
>              check_and_stop_scrub(successor);
>  
> +            /* Keep successor's first_dirty if the block doesn't have one */
> +            if ( pg->u.free.first_dirty == INVALID_DIRTY_IDX &&
> +                 successor->u.free.first_dirty != INVALID_DIRTY_IDX )
> +                pg->u.free.first_dirty = (1U << order) +
> +                                         successor->u.free.first_dirty;

Same then accordingly here (and both doable while committing,
provided you agree) and then
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan



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