linux-ext4 - Re: [PATCH] fs-verity: implement readahead for FS_IOC_ENABLE

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Date:   Wed, 4 Dec 2019 10:10:56 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Vyacheslav Dubeyko <slava@...eyko.com>
Cc:     linux-fscrypt@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org,
        Victor Hsieh <victorhsieh@...gle.com>
Subject: Re: [PATCH] fs-verity: implement readahead for FS_IOC_ENABLE_VERITY

On Wed, Dec 04, 2019 at 10:53:50AM +0300, Vyacheslav Dubeyko wrote:
> > diff --git a/fs/verity/enable.c b/fs/verity/enable.c
> > index eabc6ac19906..f7eaffa60196 100644
> > --- a/fs/verity/enable.c
> > +++ b/fs/verity/enable.c
> > @@ -13,14 +13,44 @@
> >  #include <linux/sched/signal.h>
> >  #include <linux/uaccess.h>
> >  
> > -static int build_merkle_tree_level(struct inode *inode, unsigned int
> > level,
> > +/*
> > + * Read a file data page for Merkle tree construction.  Do
> > aggressive readahead,
> > + * since we're sequentially reading the entire file.
> > + */
> > +static struct page *read_file_data_page(struct inode *inode,
> > +					struct file_ra_state *ra,
> > +					struct file *filp,
> > +					pgoff_t index,
> > +					pgoff_t num_pages_in_file)
> > +{
> > +	struct page *page;
> > +
> > +	page = find_get_page(inode->i_mapping, index);
> > +	if (!page || !PageUptodate(page)) {
> > +		if (page)
> > +			put_page(page);
> 
> 
> It looks like that there is not necessary check here. If we have NULL
> pointer on page then we will not enter inside. But if we have valid
> pointer on page then we have double check inside. Am I correct? 
> 

I'm not sure what you mean.  This code does the page_cache_sync_readahead() and
read_mapping_page() if either the page is not in the pagecache at all *or* is
not up to date.  I know this is slightly different logic than
generic_file_buffered_read() uses, and is suboptimal since the use of
read_mapping_page() causes a redundant pagecache lookup.  But we don't need to
squeeze out every possible bit of performance here.

Hmm, maybe it should only call page_cache_sync_readahead() when page == NULL
though.  I'll check the readahead code again.

> 
> > +		page_cache_sync_readahead(inode->i_mapping, ra, filp,
> > +					  index, num_pages_in_file -
> > index);
> > +		page = read_mapping_page(inode->i_mapping, index,
> > NULL);
> > +		if (IS_ERR(page))
> > +			return page;
> 
> Could we recieve the NULL pointer here? Is callee ready to process theNULL return value? 
> 

No, read_mapping_page() returns either a valid page or an ERR_PTR().

- Eric