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SubjectRe: [PATCH v6 07/22] Replace the XIP page fault handler with the DAX page fault handler
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On Fri, 2014-02-28 at 15:20 -0500, Matthew Wilcox wrote:
> On Fri, Feb 28, 2014 at 10:49:31AM -0700, Toshi Kani wrote:
> > On Tue, 2014-02-25 at 09:18 -0500, Matthew Wilcox wrote:
:
> Glad to see you're looking at it. Let me try to help ...

Hi Matt,

Thanks for the help. This is really a nice work, and I am hoping to
help it... (in some day! :-)

> > The original code,
> > xip_file_fault(), jumps to found: and calls vm_insert_mixed() when
> > get_xip_mem(,,0,,) succeeded. If get_xip_mem() returns -ENODATA, it
> > calls either get_xip_mem(,,1,,) or xip_sparse_page(). In this new
> > function, it looks to me that get_block(,,,0) returns 0 for both cases
> > (success and -ENODATA previously), which are dealt in the same way. Is
> > that right? If so, is there any reason for the change?
>
> Yes, get_xip_mem() returned -ENODATA for a hole. That was a suboptimal
> interface because filesystems are actually capable of returning more
> information than that, eg how long the hole is (ext4 *doesn't*, but I
> consider that to be a bug).
>
> I don't get to decide what the get_block() interface looks like. It's the
> standard way that the VFS calls back into the filesystem and has been
> around for probably close to twenty years at this point. I'm still trying
> to understand exactly what the contract is for get_blocks() ... I have
> a document that I'm working on to try to explain it, but it's tough going!

Got it. Yes, get_block() is a beast for file system newbie like me.
Thanks for working on the document.

> > Also, isn't it
> > possible to call get_block(,,,1) even if get_block(,,,0) found a block?
>
> The code in question looks like this:
>
> error = get_block(inode, block, &bh, 0);
> if (error || bh.b_size < PAGE_SIZE)
> goto sigbus;
>
> if (!buffer_written(&bh) && !vmf->cow_page) {
> if (vmf->flags & FAULT_FLAG_WRITE) {
> error = get_block(inode, block, &bh, 1);
>
> where buffer_written is defined as:
> return buffer_mapped(bh) && !buffer_unwritten(bh);
>
> Doing some boolean algebra, that's:
>
> if (!buffer_mapped || buffer_unwritten)

Oh, I see! When the first get_block(,,,0) succeeded, this buffer is
mapped. So, it won't go into this path.

> In either case, we want to tell the filesystem that we're writing to
> this block. At least, that's my current understanding of the get_block()
> interface. I'm open to correction here!

Thanks again!
-Toshi



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