netdev - Re: [PATCH 7/7] net: page_pool: use alloc_pages

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Date:   Fri, 12 Mar 2021 22:05:45 +0200
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-Net <netdev@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-NFS <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH 7/7] net: page_pool: use alloc_pages_bulk in refill code
 path

[...]
> 6. return last_page
> 
> > +       /* Remaining pages store in alloc.cache */
> > +       list_for_each_entry_safe(page, next, &page_list, lru) {
> > +               list_del(&page->lru);
> > +               if ((pp_flags & PP_FLAG_DMA_MAP) &&
> > +                   unlikely(!page_pool_dma_map(pool, page))) {
> > +                       put_page(page);
> > +                       continue;
> > +               }
> 
> So if you added a last_page pointer what you could do is check for it
> here and assign it to the alloc cache. If last_page is not set the
> block would be skipped.
> 
> > +               if (likely(pool->alloc.count < PP_ALLOC_CACHE_SIZE)) {
> > +                       pool->alloc.cache[pool->alloc.count++] = page;
> > +                       pool->pages_state_hold_cnt++;
> > +                       trace_page_pool_state_hold(pool, page,
> > +                                                  pool->pages_state_hold_cnt);
> > +               } else {
> > +                       put_page(page);
> 
> If you are just calling put_page here aren't you leaking DMA mappings?
> Wouldn't you need to potentially unmap the page before you call
> put_page on it?

Oops, I completely missed that. Alexander is right here.

> 
> > +               }
> > +       }
> > +out:
> >         if ((pp_flags & PP_FLAG_DMA_MAP) &&
> > -           unlikely(!page_pool_dma_map(pool, page))) {
> > -               put_page(page);
> > +           unlikely(!page_pool_dma_map(pool, first_page))) {
> > +               put_page(first_page);
> 
> I would probably move this block up and make it a part of the pp_order
> block above. Also since you are doing this in 2 spots it might make
> sense to look at possibly making this an inline function.
> 
> >                 return NULL;
> >         }
> >
> >         /* Track how many pages are held 'in-flight' */
> >         pool->pages_state_hold_cnt++;
> > -       trace_page_pool_state_hold(pool, page, pool->pages_state_hold_cnt);
> > +       trace_page_pool_state_hold(pool, first_page, pool->pages_state_hold_cnt);
> >
> >         /* When page just alloc'ed is should/must have refcnt 1. */
> > -       return page;
> > +       return first_page;
> >  }
> >
> >  /* For using page_pool replace: alloc_pages() API calls, but provide
> > --
> > 2.26.2
> >

Cheers
/Ilias