Re: [PATCH v2 24/33] mm/slob: Convert SLOB to use struct slab

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Fri, Dec 10, 2021 at 12:44:32PM +0100, Vlastimil Babka wrote:
> On 12/10/21 11:44, Hyeonggon Yoo wrote:
> > On Wed, Dec 01, 2021 at 07:15:01PM +0100, Vlastimil Babka wrote:
> >> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> >> 
> >> Use struct slab throughout the slob allocator.
> >> 
> >> [ vbabka@suse.cz: don't introduce wrappers for PageSlobFree in mm/slab.h just
> >>   for the single callers being wrappers in mm/slob.c ]
> >> 
> >> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> >> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> >> ---
> >>  mm/slob.c | 34 +++++++++++++++++-----------------
> >>  1 file changed, 17 insertions(+), 17 deletions(-)
> >> 
> >> diff --git a/mm/slob.c b/mm/slob.c
> >> index d2d15e7f191c..d3512bcc3141 100644
> >> --- a/mm/slob.c
> >> +++ b/mm/slob.c
> > 
> > ...
> > 
> >>  		/* Enough room on this page? */
> >> @@ -358,8 +358,8 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node,
> >>  		b = slob_new_pages(gfp & ~__GFP_ZERO, 0, node);
> >>  		if (!b)
> >>  			return NULL;
> >> -		sp = virt_to_page(b);
> >> -		__SetPageSlab(sp);
> >> +		sp = virt_to_slab(b);
> >> +		__SetPageSlab(slab_page(sp));
> > 
> > Hello Vlastimil.
> > 
> > I've tested this patch on my machine and it causes NULL pointer dereference.
> > that's because virt_to_slab returns NULL if folio_test_slab is false.
> > and __SetPageSlab is called with sp = NULL.
> 
> Oops, thanks for catching this.
> 
> > diff below fixed bug.
> 
> Changed it to use folio and will push an updated branch
> slab-struct_slab-v3r2
> 
> Thanks!

Tested again on slab-struct_slab-v3r2 and everyting works fine!
And the code overall looks good to me.

Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

And below is not about this patch,
but what about something like this?

Because it's not necessary that page->_mapcount and
slab->units have same offset, so we can remove
(somewhat confusing) page_mapcount_reset function call.

I want to remove calling page_mapcount_reset also in
SLAB, but I have no idea. (maybe we can do that if we remove
SLAB colouring but that's going too far)

diff --git a/mm/slab.h b/mm/slab.h
index 90d7fceba470..dd0480149d38 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -50,8 +50,8 @@ struct slab {
        struct list_head slab_list;
        void * __unused_1;
        void *freelist;         /* first free block */
-       void * __unused_2;
-       int units;
+       long units;
+       unsigned int __unused_2;

 #else
 #error "Unexpected slab allocator configured"
diff --git a/mm/slob.c b/mm/slob.c
index 39b651b3e6e7..7b2d2c7d69cc 100644
--- a/mm/slob.c
+++ b/mm/slob.c
@@ -404,7 +404,6 @@ static void slob_free(void *block, int size)
                        clear_slob_page_free(sp);
                spin_unlock_irqrestore(&slob_lock, flags);
                __ClearPageSlab(slab_page(sp));
-               page_mapcount_reset(slab_page(sp));
                slob_free_pages(b, 0);
                return;
        }

> 
> > diff --git a/mm/slob.c b/mm/slob.c
> > index d3512bcc3141..cf669f03440f 100644
> > --- a/mm/slob.c
> > +++ b/mm/slob.c
> > @@ -358,8 +358,8 @@ static void *slob_alloc(size_t size, gfp_t gfp, int a
> > lign, int node,
> >                 b = slob_new_pages(gfp & ~__GFP_ZERO, 0, node);
> >                 if (!b)
> >                         return NULL;
> > +               __SetPageSlab(virt_to_page(b));
> >                 sp = virt_to_slab(b);
> > -               __SetPageSlab(slab_page(sp));
> > 
> >                 spin_lock_irqsave(&slob_lock, flags);
> >                 sp->units = SLOB_UNITS(PAGE_SIZE);
> >  
> >  Thanks,
> >  Hyeonggon.
> > 
> >>  
> >>  		spin_lock_irqsave(&slob_lock, flags);
> >>  		sp->units = SLOB_UNITS(PAGE_SIZE);
> >> @@ -381,7 +381,7 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node,
> >>   */
> >>  static void slob_free(void *block, int size)
> >>  {
> >> -	struct page *sp;
> >> +	struct slab *sp;
> >>  	slob_t *prev, *next, *b = (slob_t *)block;
> >>  	slobidx_t units;
> >>  	unsigned long flags;
> >> @@ -391,7 +391,7 @@ static void slob_free(void *block, int size)
> >>  		return;
> >>  	BUG_ON(!size);
> >>  
> >> -	sp = virt_to_page(block);
> >> +	sp = virt_to_slab(block);
> >>  	units = SLOB_UNITS(size);
> >>  
> >>  	spin_lock_irqsave(&slob_lock, flags);
> >> @@ -401,8 +401,8 @@ static void slob_free(void *block, int size)
> >>  		if (slob_page_free(sp))
> >>  			clear_slob_page_free(sp);
> >>  		spin_unlock_irqrestore(&slob_lock, flags);
> >> -		__ClearPageSlab(sp);
> >> -		page_mapcount_reset(sp);
> >> +		__ClearPageSlab(slab_page(sp));
> >> +		page_mapcount_reset(slab_page(sp));
> >>  		slob_free_pages(b, 0);
> >>  		return;
> >>  	}
> >> -- 
> >> 2.33.1
> >> 
> >> 
>