linux-kernel - Re: write(2) and page cache

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Message-ID: <alpine.LNX.2.00.1307292242380.9718@eggly.anvils>
Date:	Mon, 29 Jul 2013 22:54:12 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	dmi p <d3vp0ll@...il.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: write(2) and page cache

On Sun, 28 Jul 2013, dmi p wrote:
> Hi,
> When I write(2) on a file such that the size of the buffer being
> written is less than the page size, and the page is not in the page
> cache, does the write syscall block until the page is brought into the
> page cache from disk (to modify it and eventually write back),

Yes (except perhaps when overwriting that part of page at end of file:
I expect some filesystems might optimize out that read, and others not).

> or is the page reading part async with respect to write?

No, that would involve another layer of buffering, which I think
would be too hard to synchronize with page cache and mmap.

Hugh
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