Hi,
While trying to figure out which things to convert from
get_user_pages*() to put_user_pages*(), I came across an interesting use
of set_page_dirty_lock(), and wanted to ask about it.
Is it safe to call set_page_dirty_lock() like this (for the case
when this is file-backed memory):
// drivers/vhost/vhost.c:1757:
static int set_bit_to_user(int nr, void __user *addr)
{
unsigned long log = (unsigned long)addr;
struct page *page;
void *base;
int bit = nr + (log % PAGE_SIZE) * 8;
int r;
r = get_user_pages_fast(log, 1, FOLL_WRITE, &page);
if (r < 0)
return r;
BUG_ON(r != 1);
base = kmap_atomic(page);
set_bit(bit, base);
kunmap_atomic(base);
set_page_dirty_lock(page);
put_page(page);
return 0;
}
?
That is, after the page is unmapped, but before unpinning it?
Specifically, I'd expect that the writeback and reclaim code code can end
up calling drop_buffers() (because the set_bit() call actually did
dirty the pte), after the kunmap_atomic() call. So then when
set_page_dirty_lock() runs, it could bug check on ext4_writepage()'s
attempt to buffer heads:
ext4_writepage()
page_bufs = page_buffers(page);
#define page_buffers(page) \
({ \
BUG_ON(!PagePrivate(page)); \
((struct buffer_head *)page_private(page)); \
})
...which actually is the the case that pin_user_pages*() is ultimately
helping to avoid, btw. But in this case, it's all code that runs on a
CPU, so no DMA or DIO is involved. But still, the "bare" use of
set_page_dirty_lock() seems like a problem here.
thanks,
--
John Hubbard
NVIDIA
