Re: [PATCH 3/3] mm: Allow find_get_page to be used for large pages

[Matthew Wilcox <willy@infradead.org>]

On Fri, Sep 06, 2019 at 03:59:28PM +0300, Kirill A. Shutemov wrote:
> > @@ -248,6 +248,15 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
> >  #define FGP_NOFS		0x00000010
> >  #define FGP_NOWAIT		0x00000020
> >  #define FGP_FOR_MMAP		0x00000040
> > +/*
> > + * If you add more flags, increment FGP_ORDER_SHIFT (no further than 25).
> 
> Maybe some BUILD_BUG_ON()s to ensure FGP_ORDER_SHIFT is sane?

Yeah, probably a good idea.

> > +/**
> > + * __find_get_page - Find and get a page cache entry.
> > + * @mapping: The address_space to search.
> > + * @offset: The page cache index.
> > + * @order: The minimum order of the entry to return.
> > + *
> > + * Looks up the page cache entries at @mapping between @offset and
> > + * @offset + 2^@order.  If there is a page cache page, it is returned with
> 
> Off by one? :P

Hah!  I thought it reasonable to be ambiguous in the English description
...  it's not entirely uncommon to describe something being 'between A
and B' when meaning ">= A and < B".

> > +static struct page *__find_get_page(struct address_space *mapping,
> > +		unsigned long offset, unsigned int order)
> > +{
> > +	XA_STATE(xas, &mapping->i_pages, offset);
> > +	struct page *page;
> > +
> > +	rcu_read_lock();
> > +repeat:
> > +	xas_reset(&xas);
> > +	page = xas_find(&xas, offset | ((1UL << order) - 1));
> 
> Hm. '|' is confusing. What is expectation about offset?
> Is round_down(offset, 1UL << order) expected to be equal offset?
> If yes, please use '+' instead of '|'.

Might make sense to put in ...

	VM_BUG_ON(offset & ((1UL << order) - 1));

> > +	if (xas_retry(&xas, page))
> > +		goto repeat;
> > +	/*
> > +	 * A shadow entry of a recently evicted page, or a swap entry from
> > +	 * shmem/tmpfs.  Skip it; keep looking for pages.
> > +	 */
> > +	if (xa_is_value(page))
> > +		goto repeat;
> > +	if (!page)
> > +		goto out;
> > +	if (compound_order(page) < order) {
> > +		page = XA_RETRY_ENTRY;
> > +		goto out;
> > +	}
> 
> compound_order() is not stable if you don't have pin on the page.
> Check it after page_cache_get_speculative().

Maybe check both before and after?  If we check it before, we don't bother
to bump the refcount on a page which is too small.

> > @@ -1632,6 +1696,10 @@ EXPORT_SYMBOL(find_lock_entry);
> >   * - FGP_FOR_MMAP: Similar to FGP_CREAT, only we want to allow the caller to do
> >   *   its own locking dance if the page is already in cache, or unlock the page
> >   *   before returning if we had to add the page to pagecache.
> > + * - FGP_PMD: We're only interested in pages at PMD granularity.  If there
> > + *   is no page here (and FGP_CREATE is set), we'll create one large enough.
> > + *   If there is a smaller page in the cache that overlaps the PMD page, we
> > + *   return %NULL and do not attempt to create a page.
> 
> Is it really the best inteface?
> 
> Maybe allow user to ask bitmask of allowed orders? For THP order-0 is fine
> if order-9 has failed.

That's the semantics that filemap_huge_fault() wants.  If the page isn't
available at order-9, it needs to return VM_FAULT_FALLBACK (and the VM
will call into filemap_fault() to handle the regular sized fault).

Now, maybe there are other users who want to specify "create a page of
this size if you can, but if there's already something there smaller,
return that".  We can add another FGP flag when those show up ;-)


Thanks for the review.