Dave Hansen wrote:
> 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.
I guess this way is better, right?
@@ -2382,6 +2393,7 @@ static ssize_t generic_perform_write(struct file *file,
unsigned long bytes; /* Bytes to write to page */
size_t copied; /* Bytes copied from user */
void *fsdata;
+ int subpage_nr = 0;
offset = (pos & (PAGE_CACHE_SIZE - 1));
bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
@@ -2411,8 +2423,14 @@ again:
if (mapping_writably_mapped(mapping))
flush_dcache_page(page);
+ if (PageTransHuge(page)) {
+ off_t huge_offset = pos & ~HPAGE_PMD_MASK;
+ subpage_nr = huge_offset >> PAGE_CACHE_SHIFT;
+ }
+
pagefault_disable();
- copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
+ copied = iov_iter_copy_from_user_atomic(page + subpage_nr, i,
+ offset, bytes);
pagefault_enable();
flush_dcache_page(page);
--
Kirill A. Shutemov
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