Re: [PATCH v2 1/2] fuse: Fix possible deadlock when writing back dirty pages

Miklos Szeredi
2021-04-14T13:20:19Z

On Wed, Apr 14, 2021 at 2:22 PM Peng Tao <bergwolf@gmail.com> wrote:
>

> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -1117,17 +1117,12 @@ static ssize_t fuse_send_write_pages(str
> > count = ia->write.out.size;
> > for (i = 0; i < ap->num_pages; i++) {
> > struct page *page = ap->pages[i];
> > + bool page_locked = ap->page_locked && (i == ap->num_pages - 1);
> Any reason for just handling the last locked page in the page array?
> To be specific, it look like the first page in the array can also be
> partial dirty and locked?

In that case the first partial page will be locked, and it'll break
out of the loop...

> >
> > - if (!err && !offset && count >= PAGE_SIZE)
> > - SetPageUptodate(page);
> > -
> > - if (count > PAGE_SIZE - offset)
> > - count -= PAGE_SIZE - offset;
> > - else
> > - count = 0;
> > - offset = 0;
> > -
> > - unlock_page(page);
> > + if (err)
> > + ClearPageUptodate(page);
> > + if (page_locked)
> > + unlock_page(page);
> > put_page(page);
> > }
> >
> > @@ -1191,6 +1186,16 @@ static ssize_t fuse_fill_write_pages(str
> > if (offset == PAGE_SIZE)
> > offset = 0;
> >
> > + /* If we copied full page, mark it uptodate */
> > + if (tmp == PAGE_SIZE)
> > + SetPageUptodate(page);
> > +
> > + if (PageUptodate(page)) {
> > + unlock_page(page);
> > + } else {
> > + ap->page_locked = true;
> > + break;

... here, and send it as a separate WRITE request.

So the multi-page case with a partial & non-uptodate head page will
always result in the write request being split into two (even if
there's no partial tail page).

Thanks,
Miklos