Re: [PATCH v6 07/22] Replace the XIP page fault handler with the DAX page fault handler

[Toshi Kani]

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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