On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> + if (PageTransHuge(page))
> + offset = pos & ~HPAGE_PMD_MASK;
> +
> pagefault_disable();
> - copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
> + copied = iov_iter_copy_from_user_atomic(
> + page + (offset >> PAGE_CACHE_SHIFT),
> + i, offset & ~PAGE_CACHE_MASK, bytes);
> pagefault_enable();
> flush_dcache_page(page);
I think there's enough voodoo in there to warrant a comment or adding
some temporary variables. There are three things going on that you wan
to convey:
1. Offset is normally <PAGE_SIZE, but you make it <HPAGE_PMD_SIZE if
you are dealing with a huge page
2. (offset >> PAGE_CACHE_SHIFT) is always 0 for small pages since
offset < PAGE_SIZE
3. "offset & ~PAGE_CACHE_MASK" does nothing for small-page offsets, but
it turns a large-page offset back in to a small-page-offset.
I think you can do it with something like this:
int subpage_nr = 0;
off_t smallpage_offset = offset;
if (PageTransHuge(page)) {
// we transform 'offset' to be offset in to the huge
// page instead of inside the PAGE_SIZE page
offset = pos & ~HPAGE_PMD_MASK;
subpage_nr = (offset >> PAGE_CACHE_SHIFT);
}
> + copied = iov_iter_copy_from_user_atomic(
> + page + subpage_nr,
> + i, smallpage_offset, bytes);
> @@ -2437,6 +2453,7 @@ again:
> * because not all segments in the iov can be copied at
> * once without a pagefault.
> */
> + offset = pos & ~PAGE_CACHE_MASK;
Urg, and now it's *BACK* in to a small-page offset?
This means that 'offset' has two _different_ meanings and it morphs
between them during the function a couple of times. That seems very
error-prone to me.
> bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
> iov_iter_single_seg_count(i));
> goto again;
>
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