Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > I don't think "write_one_page" sounds like a particularly good new
> > API addition.
>
> I also thing it's not a nice one. I still haven't seen a really good
> explanation of why it can't just use plain ->write
->write requires:
(1) A struct file. Refer to the ENFILE problem that my patch to do this
raised. The problem is that doing a tar of, say, a kernel tree
immediately opens ~30000 files internally, and then userspace falls over
with ENFILE all over the place. My tests multiply that by 8.
You (at least I think it was you) refused to countenance allocating file
structs internally to the kernel that weren't accounted.
(2) A pointer to a buffer. That means kmapping a page for the duration of
the write.
->write is quite heavy. For something like ext3 it goes down through quite a
few layers, in and out of the fs, VM and VFS. All this takes both time and
stack space.
I want to write a whole page from the kernel at a page-boundary in the file.
That means it is possible to use an extremely optimised aop to do that.
Really, what I want to do is have the filesystem issue its BIOs directly on
the netfs page, but the Ext3 people I talked to at the time thought that would
be too difficult to track.
David
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