Re: [RFC 0/8] Replacing the readpages a_op

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On 13 Jan 2020, at 12:40, Matthew Wilcox wrote:

> On Mon, Jan 13, 2020 at 04:42:10PM +0000, Chris Mason wrote:
>
> I did do a couple of helpers for lists for iomap before deciding the
> whole thing was too painful.  I didn't look at btrfs until just now, 
> but, um ...
>
> int extent_readpages(struct address_space *mapping, struct list_head 
> *pages,
>                      unsigned nr_pages)
> ..
>         struct page *pagepool[16];
> ..
>         while (!list_empty(pages)) {
> ..
>                         list_del(&page->lru);
>                         if (add_to_page_cache_lru(page, mapping, 
> page->index,
> ..
>                         pagepool[nr++] = page;
>
> you're basically doing exactly what i'm proposing to be the new 
> interface!
> OK, you get one extra page per batch ;-P

This is true, I didn't explain that part well ;)  Depending on 
compression etc we might end up poking the xarray inside the actual IO 
functions, but the main difference is that btrfs is building a single 
bio.  You're moving the plug so you'll merge into single bio, but I'd 
rather build 2MB bios than merge them.

I guess it doesn't feel like enough of a win to justify the churn.  If 
we find a way to do much larger pagevecs, I think this makes more sense.

-chris





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