Re: [PATCH v2 5/9] iomap: Support arbitrarily many blocks per page

[Qian Cai]

On Tue, 2020-09-22 at 18:05 +0100, Matthew Wilcox wrote:
> On Tue, Sep 22, 2020 at 12:23:45PM -0400, Qian Cai wrote:
> > On Fri, 2020-09-11 at 00:47 +0100, Matthew Wilcox (Oracle) wrote:
> > > Size the uptodate array dynamically to support larger pages in the
> > > page cache.  With a 64kB page, we're only saving 8 bytes per page today,
> > > but with a 2MB maximum page size, we'd have to allocate more than 4kB
> > > per page.  Add a few debugging assertions.
> > > 
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
> > 
> > Some syscall fuzzing will trigger this on powerpc:
> > 
> > .config: https://gitlab.com/cailca/linux-mm/-/blob/master/powerpc.config
> > 
> > [ 8805.895344][T445431] WARNING: CPU: 61 PID: 445431 at fs/iomap/buffered-
> > io.c:78 iomap_page_release+0x250/0x270
> 
> Well, I'm glad it triggered.  That warning is:
>         WARN_ON_ONCE(bitmap_full(iop->uptodate, nr_blocks) !=
>                         PageUptodate(page));
> so there was definitely a problem of some kind.
> 
> truncate_cleanup_page() calls
> do_invalidatepage() calls
> iomap_invalidatepage() calls
> iomap_page_release()
> 
> Is this the first warning?  I'm wondering if maybe there was an I/O error
> earlier which caused PageUptodate to get cleared again.  If it's easy to
> reproduce, perhaps you could try something like this?
> 
> +void dump_iomap_page(struct page *page, const char *reason)
> +{
> +       struct iomap_page *iop = to_iomap_page(page);
> +       unsigned int nr_blocks = i_blocks_per_page(page->mapping->host, page);
> +
> +       dump_page(page, reason);
> +       if (iop)
> +               printk("iop:reads %d writes %d uptodate %*pb\n",
> +                               atomic_read(&iop->read_bytes_pending),
> +                               atomic_read(&iop->write_bytes_pending),
> +                               nr_blocks, iop->uptodate);
> +       else
> +               printk("iop:none\n");
> +}
> 
> and then do something like:
> 
> 	if (bitmap_full(iop->uptodate, nr_blocks) != PageUptodate(page))
> 		dump_iomap_page(page, NULL);

This:

[ 1683.158254][T164965] page:000000004a6c16cd refcount:2 mapcount:0 mapping:00000000ea017dc5 index:0x2 pfn:0xc365c
[ 1683.158311][T164965] aops:xfs_address_space_operations ino:417b7e7 dentry name:"trinity-testfile2"
[ 1683.158354][T164965] flags: 0x7fff8000000015(locked|uptodate|lru)
[ 1683.158392][T164965] raw: 007fff8000000015 c00c0000019c4b08 c00c0000019a53c8 c000201c8362c1e8
[ 1683.158430][T164965] raw: 0000000000000002 0000000000000000 00000002ffffffff c000201c54db4000
[ 1683.158470][T164965] page->mem_cgroup:c000201c54db4000
[ 1683.158506][T164965] iop:none