linux-kernel - Re: [RFC PATCH 1/2] mm: introduce bmap

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Date:   Tue, 20 Jun 2017 09:34:56 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        linux-api@...r.kernel.org, Dave Chinner <david@...morbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, Jeff Moyer <jmoyer@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [RFC PATCH 1/2] mm: introduce bmap_walk()

On Mon, Jun 19, 2017 at 07:19:57PM +0100, Al Viro wrote:
> Speaking of iomap, what's supposed to happen when doing a write into what
> used to be a hole?  Suppose we have a file with a megabyte hole in it
> and there's some process mmapping that range.  Another process does
> write over the entire range.  We call ->iomap_begin() and allocate
> disk blocks.  Then we start copying data into those.  In the meanwhile,
> the first process attempts to fetch from address in the middle of that
> hole.  What should happen?

Right now the buffered iomap code expects delayed allocations.
So ->iomap_begin will only reserve block in memory, and not even
mark the blocks as allocated in the page / buffer_head.  The fact
that the block is allocated is only propagated into the page buffer_head
on a page by page basis in the actor.

> Should the blocks we'd allocated in ->iomap_begin() be immediately linked
> into the whatever indirect locks/btree/whatnot we are using?  That would
> require zeroing all of them first - otherwise that readpage will read
> uninitialized block.  Another variant would be to delay linking them
> in until ->iomap_end(), but...  Suppose we get the page evicted by
> memory pressure after the writer is finished with it.  If ->readpage()
> comes before ->iomap_end(), we'll need to somehow figure out that it's
> not a hole anymore, or we'll end up with an uptodate page full of zeroes
> observed by reads after successful write().

Delayed blocks are ignored by the read code, so it will read 'through'
them.

> The comment you've got in linux/iomap.h would seem to suggest the second
> interpretation, but neither it nor anything in Documentation discusses the
> relations with readpage/writepage...

I'll see if I can come up with some better documentation.