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    SubjectRe: replace "memset(...,0,PAGE_SIZE)" calls with "clear_page()"?
    On Mon, 1 Jan 2007, Paul Mundt wrote:

    > On Sat, Dec 30, 2006 at 06:04:14PM -0500, Robert P. J. Day wrote:
    > > fair enough. *technically*, not every call of the form
    > > "memset(ptr,0,PAGE_SIZE)" necessarily represents an address that's on
    > > a page boundary. but, *realistically*, i'm guessing most of them do.
    > > just grabbing a random example from some grep output:
    > >
    > > arch/sh/mm/init.c:
    > > ...
    > > /* clear the zero-page */
    > > memset(empty_zero_page, 0, PAGE_SIZE);
    > > ...
    > >
    > The problem with random grepping is that it doesn't give you any
    > context. clear_page() isn't available in this case since we have a
    > couple of different ways of implementing it, and the optimal
    > approach is selected later on. There are also additional assumptions
    > regarding alignment that don't allow clear_page() to be used
    > directly as replacement for the memset() callsites (as has already
    > been pointed out for some of the other architectures). While the
    > empty_zero_page in this case sits on a full page boundary, others do
    > not.
    >
    > You might find some places in drivers that do this where you might
    > be able to optimize things slightly with a clear_page() (or
    > copy_page() in the memcpy() case), but it's going to need a lot of
    > manual auditing rather than a find and replace. Any sort of wins you
    > get out of this would be marginal at best, anyways.
    >
    > The more interesting case would be page clustering/bulk page
    > clearing with offload engines, and there's certainly room to build
    > on the SGI patches for this.

    your point is well taken -- i wasn't trying to suggest that a blind
    cut-and-replace would be appropriate, only that there were an awful
    lot of places where it wasn't clear that that kind of replacement
    *wasn't* appropriate. or perhaps even recommended. (doing that kind
    of search in the drivers/ directory would perhaps be more meaningful
    than in the arch/ directory. just my luck i picked a bad example.)

    clearly, that kind of replacement might require manual intervention in
    a lot of cases, no question. as with other examples i've brought up
    here, i'm just looking at this from a relatively newbie perspective,
    where i'm perusing the code and, in this case, got to thinking, "gee,
    given that every architecture defines a clear_page() macro, i wonder
    why all these people keep calling memset()." that's all.

    kind of like how, given that include/linux/gfp.h defines the macro
    __get_dma_pages(), so many people persist in calling
    __get_free_pages() with a GFP_DMA setting. that sort of thing. :-)

    rday
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