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

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

On Tue, 2018-07-10 at 11:34 -0400, Boris Ostrovsky wrote:
> On 07/10/2018 11:15 AM, Jan Beulich wrote:
> > > > > 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.
> 
> "Keep" was used here for the (not provided) "else" clause. But I can see
> how it can be confusing.
> 
> "Update predecessor's first_dirty if necessary"? Or maybe even drop it.

I'd like to retain the comments. Personally, I like the following
variant because the if statement logic is pretty self-explanatory:

    /* Update predecessor's first_dirty if necessary */
    ...
    /* Update pg's first_dirty if necessary */
    
These changes can be done while committing.

-- 
Thanks,
Sergey
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