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

Pankaj Raghav (Samsung)
2024-06-11T09:41:51Z

> > 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