Re: PG_updatodate vs BH_updatodate

["Francis Moreau" <francis.moro@gmail.com>]

On Mon, Nov 24, 2008 at 4:56 PM, Jörn Engel <joern@logfs.org> wrote:
> On Sun, 23 November 2008 21:14:47 +0100, Francis Moreau wrote:
>> Matthew Wilcox <matthew@wil.cx> writes:
>> > On Sun, Nov 23, 2008 at 01:14:52PM +0100, Francis Moreau wrote:
>> >
>> >> Are there any cases where a page can be partially uptodate ?
>> >
>> > Consider a filesystem with 1k blocks and a system with a page size of 4k.
>> > You have a buffer_head for each of the four blocks that are being kept
>> > in the page, and you want to track their dirty state independently.
>>
>> Sorry but I'm confused since you're taking about the dirty state
>> (tracked by BH_Dirty bit) and I was taking about the uptodate state
>> (tracked by BH_Uptodate bit).
>
> Think page cache, except that the granularity is not pages but 1k
> blocks.  If your filesystem wants to read an indirect block, 1k is read
> into the cache, the other 3k (or 63k) of the page remain as they were.
>

So if I understand you correctly, we can update a page cache partially
only when the
filesystems needs to read its metadata. But not when reading a page of data.

That's pretty what Matthew answered I think but I'm confused about the
dirty bit of the
buffer header he mentioned.

Thanks
-- 
Francis
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