linux-kernel - Re: Folio discussion recap

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Date:   Thu, 23 Sep 2021 04:41:04 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: Folio discussion recap

On Wed, Sep 22, 2021 at 05:45:15PM -0700, Ira Weiny wrote:
> On Tue, Sep 21, 2021 at 11:18:52PM +0100, Matthew Wilcox wrote:
> > +/**
> > + * page_slab - Converts from page to slab.
> > + * @p: The page.
> > + *
> > + * This function cannot be called on a NULL pointer.  It can be called
> > + * on a non-slab page; the caller should check is_slab() to be sure
> > + * that the slab really is a slab.
> > + *
> > + * Return: The slab which contains this page.
> > + */
> > +#define page_slab(p)		(_Generic((p),				\
> > +	const struct page *:	(const struct slab *)_compound_head(p), \
> > +	struct page *:		(struct slab *)_compound_head(p)))
> > +
> > +static inline bool is_slab(struct slab *slab)
> > +{
> > +	return test_bit(PG_slab, &slab->flags);
> > +}
> > +
> 
> I'm sorry, I don't have a dog in this fight and conceptually I think folios are
> a good idea...
> 
> But for this work, having a call which returns if a 'struct slab' really is a
> 'struct slab' seems odd and well, IMHO, wrong.  Why can't page_slab() return
> NULL if there is no slab containing that page?

No, this is a good question.

The way slub works right now is that if you ask for a "large" allocation,
it does:

        flags |= __GFP_COMP;
        page = alloc_pages_node(node, flags, order);

and returns page_address(page) (eventually; the code is more complex)
So when you call kfree(), it uses the PageSlab flag to determine if the
allocation was "large" or not:

        page = virt_to_head_page(x);
        if (unlikely(!PageSlab(page))) {
                free_nonslab_page(page, object);
                return;
        }
        slab_free(page->slab_cache, page, object, NULL, 1, _RET_IP_);

Now, you could say that this is a bad way to handle things, and every
allocation from slab should have PageSlab set, and it should use one of
the many other bits in page->flags to indicate whether it's a large
allocation or not.  I may have feelings in that direction myself.
But I don't think I should be changing that in this patch.

Maybe calling this function is_slab() is the confusing thing.
Perhaps it should be called SlabIsLargeAllocation().  Not sure.