I think what might be happening is that pdflush writes them out fine,
however we don't trap writes by the application _during_ that writeout.
Yeah. I believe that more exactly it happens if the very last
write to the page causes a writeback (due to dirty balancing)
while another writeback for the page is already happening.
As usual in these cases, I have zero proof.
It's something that will only occur with writeback and MAP_SHARED
writable access to the file pages.
It's the do_wp_page -> balance_dirty_pages -> generic_writepages
path for sure. Maybe it's enough to change
if (wbc->sync_mode != WB_SYNC_NONE)
wait_on_page_writeback(page);
if (PageWriteback(page) ||
!clear_page_dirty_for_io(page)) {
unlock_page(page);
continue;
}
to
if (wbc->sync_mode != WB_SYNC_NONE)
wait_on_page_writeback(page);
if (PageWriteback(page)) {
redirty_page_for_writepage(wbc, page);
unlock_page(page);
continue;
}
if (!clear_page_dirty_for_io(page)) {
unlock_page(page);
continue;
}
or something along those lines. Completely untested of course :-)
Segher
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