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SubjectRe: [PATCHv3 15/41] filemap: handle huge pages in do_generic_file_read()
On Tue, Nov 01, 2016 at 05:39:40PM +0100, Jan Kara wrote:
> I'd also note that having PMD-sized pages has some obvious disadvantages as
> well:
>
> 1) I'm not sure buffer head handling code will quite scale to 512 or even
> 2048 buffer_heads on a linked list referenced from a page. It may work but
> I suspect the performance will suck.

buffer_head handling always sucks. For the iomap based bufferd write
path I plan to support a buffer_head-less mode for the block size ==
PAGE_SIZE case in 4.11 latest, but if I get enough other things of my
plate in time even for 4.10. I think that's the right way to go for
THP, especially if we require the fs to allocate the whole huge page
as a single extent, similar to the DAX PMD mapping case.

> 2) PMD-sized pages result in increased space & memory usage.

How so?

> 3) In ext4 we have to estimate how much metadata we may need to modify when
> allocating blocks underlying a page in the worst case (you don't seem to
> update this estimate in your patch set). With 2048 blocks underlying a page,
> each possibly in a different block group, it is a lot of metadata forcing
> us to reserve a large transaction (not sure if you'll be able to even
> reserve such large transaction with the default journal size), which again
> makes things slower.

As said above I think we should only use huge page mappings if there is
a single underlying extent, same as in DAX to keep the complexity down.

> 4) As you have noted some places like write_begin() still depend on 4k
> pages which creates a strange mix of places that use subpages and that use
> head pages.

Just use the iomap bufferd I/O code and all these issues will go away.

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