linux-kernel - Re: [PATCH v7 07/11] iomap: fix iomap_dio

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Date: Tue, 11 Jun 2024 09:41:37 +0000
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: John Garry <john.g.garry@...cle.com>
Cc: david@...morbit.com, djwong@...nel.org, chandan.babu@...cle.com,
	brauner@...nel.org, akpm@...ux-foundation.org, willy@...radead.org,
	mcgrof@...nel.org, linux-mm@...ck.org, hare@...e.de,
	linux-kernel@...r.kernel.org, yang@...amperecomputing.com,
	Zi Yan <zi.yan@...t.com>, linux-xfs@...r.kernel.org,
	p.raghav@...sung.com, linux-fsdevel@...r.kernel.org, hch@....de,
	gost.dev@...sung.com, cl@...amperecomputing.com
Subject: Re: [PATCH v7 07/11] iomap: fix iomap_dio_zero() for fs bs > system
 page size

> > index 49938419fcc7..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? ;)

Do you have something like this in mind?

diff --git a/fs/internal.h b/fs/internal.h
index 30217f0ff4c6..def96c7ed9ea 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -39,6 +39,7 @@ int __block_write_begin_int(struct folio *folio, loff_t pos, unsigned len,
  * iomap/direct-io.c
  */
 int iomap_dio_init(void);
+void iomap_dio_exit(void);
 
 /*
  * char_dev.c
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 9f791db473e4..8d8b9e62201f 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -1994,10 +1994,16 @@ static int __init iomap_init(void)
 
        ret = iomap_dio_init();
        if (ret)
-               return ret;
+               goto out;
 
-       return bioset_init(&iomap_ioend_bioset, 4 * (PAGE_SIZE / SECTOR_SIZE),
+       ret = bioset_init(&iomap_ioend_bioset, 4 * (PAGE_SIZE / SECTOR_SIZE),
                           offsetof(struct iomap_ioend, io_bio),
                           BIOSET_NEED_BVECS);
+       if (!ret)
+               goto out;
+
+       iomap_dio_exit();
+out:
+       return ret;
 }
 fs_initcall(iomap_init);
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index b95600b254a3..f4c9445ca50d 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -69,6 +69,12 @@ int iomap_dio_init(void)
        return 0;
 }
 
+void iomap_dio_exit(void)
+{
+       __free_pages(zero_fs_block, ZERO_FSB_ORDER);
+
+}
+
 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 in https://lore.kernel.org/linux-xfs/20240429174746.2132161-1-john.g.garry@oracle.com/T/#m5354e2b2531a5552a8b8acd4a95342ed4d7500f2,
> 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?

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.

[1] https://lore.kernel.org/linux-xfs/20240529134509.120826-8-kernel@pankajraghav.com/

--
Pankaj