Re: [PATCH v7 07/11] iomap: fix iomap_dio_zero() for fs bs > system page size

[John Garry]

On 11/06/2024 10:41, Pankaj Raghav (Samsung) wrote:
> > > 8419fcc7..9f791db473e4 100644
> > > --- a/fs/iomap/buffered-io.c
> > > +++ b/fs/iomap/buffered-io.c
> > > @@ -1990,6 +1990,12 @@ EXPORT_SYMBOL_GPL(iomap_writepages);
> > >    static int __init iomap_init(void)
> > >    {
> > > +	int ret;
> > > +
> > > +	ret = iomap_dio_init();
> > > +	if (ret)
> > > +		return ret;
> > > +
> > >    	return bioset_init(&iomap_ioend_bioset, 4 * (PAGE_SIZE / SECTOR_SIZE),
> > >    			   offsetof(struct iomap_ioend, io_bio),
> > >    			   BIOSET_NEED_BVECS);
> > I suppose that it does not matter that zero_fs_block is leaked if this fails
> > (or is it even leaked?), as I don't think that failing that bioset_init()
> > call is handled at all.
> If bioset_init fails, then we have even more problems than just a leaked
> 64k memory? 😉

Right

> Do you have something like this in mind?

I wouldn't propose anything myself. AFAICS, we don't gracefully handle 
bioset_init() failing and iomap_ioend_bioset not being initialized properly.

>   static struct bio *iomap_dio_alloc_bio(const struct iomap_iter *iter,
>                  struct iomap_dio *dio, unsigned short nr_vecs, blk_opf_t opf)
>   {
>
> > > +
> > >    static struct bio *iomap_dio_alloc_bio(const struct iomap_iter *iter,
> > >    		struct iomap_dio *dio, unsigned short nr_vecs, blk_opf_t opf)
> > >    {
> > > @@ -236,17 +253,22 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
> > >    		loff_t pos, unsigned len)
> > >    {
> > >    	struct inode *inode = file_inode(dio->iocb->ki_filp);
> > > -	struct page *page = ZERO_PAGE(0);
> > >    	struct bio *bio;
> > > +	/*
> > > +	 * Max block size supported is 64k
> > > +	 */
> > > +	WARN_ON_ONCE(len > ZERO_FSB_SIZE);
> > JFYI, As mentioned inhttps://urldefense.com/v3/__https://lore.kernel.org/linux-xfs/20240429174746.2132161-1-john.g.garry@xxxxxxxxxx/T/*m5354e2b2531a5552a8b8acd4a95342ed4d7500f2__;Iw!!ACWV5N9M2RV99hQ!MTwVaC6oueHR_vgmDfOvgBX8bPdeTSRPcRcw5-CqtHnFEH-Ya1sUeZwaF-xrBF5XZ_8lJw5l-riq4t8IkfBhf2Q$  ,
> > we would like to support an arbitrary size. Maybe I will need to loop for
> > zeroing sizes > 64K.
> The initial patches were looping with a ZERO_PAGE(0), but the initial
> feedback was to use a huge zero page. But when I discussed that at LSF,
> the people thought we will be using a lot of memory for sub-block
> memory, especially on architectures with 64k base page size.
>
> So for now a good tradeoff between memory usage and efficiency was to
> use a 64k buffer as that is the maximum FSB we support.[1]
>
> IIUC, you will be using this function also to zero out the extent and
> not just a FSB?

Right. Or more specifically, the FS can ask for the zeroing size. 
Typically it will be inode i_blocksize, but the FS can ask for a larger 
size to zero out to extent alignment boundaries.

> I think we could resort to looping until we have a way to request
> arbitrary zero folios without having to allocate at it in
> iomap_dio_alloc_bio() for every IO.

ok

> [1]https://urldefense.com/v3/__https://lore.kernel.org/linux-xfs/20240529134509.120826-8-kernel@xxxxxxxxxxxxxxxx/__;!!ACWV5N9M2RV99hQ!MTwVaC6oueHR_vgmDfOvgBX8bPdeTSRPcRcw5-CqtHnFEH-Ya1sUeZwaF-xrBF5XZ_8lJw5l-riq4t8Ij2hl9yU$  

Thanks,
John