Re: [PATCH v3 3/3] squashfs: implement readahead

[Hsin-Yi Wang <hsinyi@chromium.org>]

On Wed, Jun 1, 2022 at 9:08 AM Phillip Lougher <phillip@squashfs.org.uk> wrote:
>
> On 31/05/2022 21:47, Andrew Morton wrote:
> > On Mon, 23 May 2022 14:59:13 +0800 Hsin-Yi Wang <hsinyi@chromium.org> wrote:
> >
> >> Implement readahead callback for squashfs. It will read datablocks
> >> which cover pages in readahead request. For a few cases it will
> >> not mark page as uptodate, including:
> >> - file end is 0.
> >> - zero filled blocks.
> >> - current batch of pages isn't in the same datablock or not enough in a
> >>    datablock.
> >> - decompressor error.
> >> Otherwise pages will be marked as uptodate. The unhandled pages will be
> >> updated by readpage later.
> >>
> >> ...
> >>
> >
> > The choice of types seems somewhat confused.
> >
> >> @@ -495,7 +496,95 @@ static int squashfs_read_folio(struct file *file, struct folio *folio)
> >>      return 0;
> >>   }
> >>
> >> +static void squashfs_readahead(struct readahead_control *ractl)
> >> +{
> >> +    struct inode *inode = ractl->mapping->host;
> >> +    struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
> >> +    size_t mask = (1UL << msblk->block_log) - 1;
> >> +    size_t shift = msblk->block_log - PAGE_SHIFT;
> >
> > block_log is unsigned short.  Why size_t?

Will update in the next version.

> >
> >> +    loff_t start = readahead_pos(ractl) &~ mask;
> >> +    size_t len = readahead_length(ractl) + readahead_pos(ractl) - start;
> >> +    struct squashfs_page_actor *actor;
> >> +    unsigned int nr_pages = 0;
> >
> > OK.
> >
> >> +    struct page **pages;
> >> +    u64 block = 0;
> >> +    int bsize, res, i, index, bytes, expected;
> >
> > `res' could be local to the inner loop.
> >
> > `i' is used in situations where an unsigned type would be more
> > appropriate.  If it is made unsigned then `i' is no longer a suitable
> > identifier.  Doesn't matter much.
> >
> > `index' is from page.index, which is pgoff_t.
> >
> > `bytes' could be local to the innermost loop.
> >
> > `expected' is inappropriately a signed type and could be local to the
> > inner loop.

Will update them in the next version.
> >
> >> +    int file_end = i_size_read(inode) >> msblk->block_log;
> >> +    unsigned int max_pages = 1UL << shift;
> >> +    void *pageaddr;
> >> +
>
> pageaddr could be made local to the innermost scope.
>
Will update them in the next version.

Thanks for your comments.

> Apart from that the patch and updated error handling looks
> good.
>
> Phillip
>
> >> +    readahead_expand(ractl, start, (len | mask) + 1);
> >> +
> >> +    if (file_end == 0)
> >> +            return;
> >> +
> >> +    pages = kmalloc_array(max_pages, sizeof(void *), GFP_KERNEL);
> >> +    if (!pages)
> >> +            return;
> >> +
> >> +    actor = squashfs_page_actor_init_special(pages, max_pages, 0);
> >> +    if (!actor)
> >> +            goto out;
> >> +
> >> +    for (;;) {
> >> +            nr_pages = __readahead_batch(ractl, pages, max_pages);
> >> +            if (!nr_pages)
> >> +                    break;
> >> +
> >> +            if (readahead_pos(ractl) >= i_size_read(inode) ||
> >> +                nr_pages < max_pages)
> >> +                    goto skip_pages;
> >> +
> >> +            index = pages[0]->index >> shift;
> >> +            if ((pages[nr_pages - 1]->index >> shift) != index)
> >> +                    goto skip_pages;
> >> +
> >> +            expected = index == file_end ?
> >> +                       (i_size_read(inode) & (msblk->block_size - 1)) :
> >> +                        msblk->block_size;
> >> +
> >> +            bsize = read_blocklist(inode, index, &block);
> >> +            if (bsize == 0)
> >> +                    goto skip_pages;
> >> +
> >> +            res = squashfs_read_data(inode->i_sb, block, bsize, NULL,
> >> +                                     actor);
> >> +
> >> +            if (res == expected) {
> >> +                    /* Last page may have trailing bytes not filled */
> >> +                    bytes = res % PAGE_SIZE;
> >> +                    if (bytes) {
> >> +                            pageaddr = kmap_atomic(pages[nr_pages - 1]);
> >> +                            memset(pageaddr + bytes, 0, PAGE_SIZE - bytes);
> >> +                            kunmap_atomic(pageaddr);
> >> +                    }
> >> +
> >> +                    for (i = 0; i < nr_pages; i++)
> >> +                            SetPageUptodate(pages[i]);
> >> +            }
> >
> > res == -EIO is unhandled?
> >
> >> +            for (i = 0; i < nr_pages; i++) {
> >> +                    unlock_page(pages[i]);
> >> +                    put_page(pages[i]);
> >> +            }
> >> +    }
> >> +
> >> +    kfree(actor);
> >> +    kfree(pages);
> >> +    return;
> >> +
> >> +skip_pages:
> >> +    for (i = 0; i < nr_pages; i++) {
> >> +            unlock_page(pages[i]);
> >> +            put_page(pages[i]);
> >> +    }
> >> +
> >> +    kfree(actor);
> >> +out:
> >> +    kfree(pages);
> >> +}
> >>
> >>   const struct address_space_operations squashfs_aops = {
> >> -    .read_folio = squashfs_read_folio
> >> +    .read_folio = squashfs_read_folio,
> >> +    .readahead = squashfs_readahead
> >>   };
> >
>