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    SubjectRe: allowed pages in the block later, was Re: [Ext2-devel] [PATCH] ext3: avoid sending down non-refcounted pages
    Mike Christie wrote:
    > Christoph Hellwig wrote:
    >> On Thu, Dec 08, 2005 at 03:18:33AM -0700, Andreas Dilger wrote:
    >>> What happens on 1kB or 2kB block filesystems (i.e. b_size != PAGE_SIZE)?
    >>> This will allocate a whole page for each block (which may be
    >>> considerable
    >>> overhead on e.g. a 64kB PAGE_SIZE ia64 or PPC system).
    >> Yes. How often do we trigger this codepath?
    >> The problem we're trying to solve here is how do implement network block
    >> devices (nbd, iscsi) efficiently. The zero copy codepath in the
    >> networking
    >> layer does need to grab additional references to pages. So to use
    >> sendpage
    >> we need a refcountable page. pages used by the slab allocator are not
    >> normally refcounted so try to do get_page/pub_page on them will break.
    >> One way to work around that would be to detect kmalloced pages and use
    >> a slowpath for that. The major issues with that is that we don't have a
    >> reliable way to detect if a given struct page comes from the slab
    >> allocator
    >> or not. The minor problem is that even with such an indicator it means
    >> having a separate and lightly tested slowpath for this rare case.
    >> All in all I think we should document that the block layer only accepts
    >> properly refcounted pages, which is everything but kmalloced pages (even
    >> vmalloc is totally fine)
    > Is it anytime kmalloc is used? For scsi when it uses scsi_execute* for
    > something like scanning (report luns result is kmallocd) would this be a
    > problem?
    > If PageSlab() does work, then could we have a request queue flag that
    > bounces those pages for all block layer drivers. Pretty slow and yucky
    > but if we have to convert SCSI and maybe other parts of the block layer
    > maybe it will be easiest for now.

    Or there is not a way to do kmalloc(GFP_BLK) that gives us the right
    type of memory is there?
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      Last update: 2005-12-08 19:25    [W:0.022 / U:86.688 seconds]
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