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    SubjectRe: [PATCH 11/11] afs: Fix dirty-region encoding on ppc32 with 64K pages
    Date
    Matthew Wilcox <willy@infradead.org> wrote:

    > > > > +{
    > > > > + if (PAGE_SIZE - 1 <= __AFS_PAGE_PRIV_MASK)
    > > > > + return 1;
    > > > > + else
    > > > > + return PAGE_SIZE / (__AFS_PAGE_PRIV_MASK + 1);
    > > >
    > > > Could this be DIV_ROUND_UP(PAGE_SIZE, __AFS_PAGE_PRIV_MASK + 1); avoiding
    > > > a conditional? I appreciate it's calculated at compile time today, but
    > > > it'll be dynamic with THP.
    > >
    > > Hmmm - actually, I want a shift size, not a number of bytes as I divide by it
    > > twice in afs_page_dirty().
    >
    > __AFS_PAGE_PRIV_MASK is a constant though. If your compiler can't
    > optimise a divide-by-a-constant-power-of-two into a shift-by-a-constant (*),
    > it's time to get yourself a new compiler.
    >
    > (*) assuming that's faster on the CPU it's targetting.

    I'm dividing by the resolution, which is calculated from the page size - which
    in a THP world is variable:

    static inline unsigned long afs_page_dirty(size_t from, size_t to)
    {
    size_t res = afs_page_dirty_resolution();
    from /= res; /* Round down */
    to = (to - 1) / res; /* Round up */
    return ((unsigned long)to << __AFS_PAGE_PRIV_SHIFT) | from;
    }

    David

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