On Fri, 9 Feb 2007 11:14:55 -0800 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 8 Feb 2007 14:07:46 +0100 (CET) Nick Piggin <npiggin@xxxxxxx> wrote:
>
> > +void page_zero_new_buffers(struct page *page, unsigned from, unsigned to)
> > +{
> > + unsigned int block_start, block_end;
> > + struct buffer_head *head, *bh;
> > +
> > + BUG_ON(!PageLocked(page));
> > + if (!page_has_buffers(page))
> > + return;
> > +
> > + bh = head = page_buffers(page);
> > + block_start = 0;
> > + do {
> > + block_end = block_start + bh->b_size;
> > +
> > + if (buffer_new(bh)) {
> > + if (block_end > from && block_start < to) {
> > + if (!PageUptodate(page)) {
> > + unsigned start, end;
> > + void *kaddr;
> > +
> > + start = max(from, block_start);
> > + end = min(to, block_end);
> > +
> > + kaddr = kmap_atomic(page, KM_USER0);
> > + memset(kaddr+start, 0, block_end-end);
> > + flush_dcache_page(page);
> > + kunmap_atomic(kaddr, KM_USER0);
> > + set_buffer_uptodate(bh);
> > + }
>
> I don't see how this differs from the previous attempts to solve the
> deadlock via atomic copt_from_user(). Here we temporarily zero out the
> pagecache page then block_perform_write() unlocks the page. So another
> thread can come in, read the page and see the temporary zeroes?
>
> If so, that might be preventable by leaving the buffer nonuptodate.
oh, OK, it was buffer_new(), so zeroes are the right thing for a reader to
see.
But if it wasn't buffer_new() then the appropriate thing for the reader to
see is what's on the disk. But __block_prepare_write() won't read a buffer
which is fully-inside the write area from disk.
And that's seemingly OK, because if a reader gets in there after the short
copy, that reader will see the non-uptodate buffer and will populate it
from disk.
But doing that will overwrite the data which the write() caller managed to
copy into the page before it took a fault. And that's not OK because
block_perform_write() does iovec_iterator_advance(i, copied) in this case
and hence will not rerun the copy after acquiring the page lock?
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