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

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On 10/27/23 17:41, Pankaj Raghav wrote:
On 27/10/2023 12:47, Matthew Wilcox wrote:
On Fri, Oct 27, 2023 at 10:03:15AM +0200, Pankaj Raghav wrote:
I also noticed this pattern in fscrypt_zeroout_range_inline_crypt().
Probably there are more places which could use a ZERO_FOLIO directly
instead of iterating with ZERO_PAGE.

Chinner also had a similar comment. It would be nice if we can reserve
a zero huge page that is the size of MAX_PAGECACHE_ORDER and add it as
one folio to the bio.

i'm on holiday atm.  start looking at mm_get_huge_zero_page()

Thanks for the pointer. I made a rough version of how it might
look like if I use that API:

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index bcd3f8cf5ea4..6ae21bd16dbe 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -236,17 +236,43 @@ 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 page *page = NULL;
+       bool huge_page = false;
+       bool fallback = false;
         struct bio *bio;

-       bio = iomap_dio_alloc_bio(iter, dio, 1, REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
+       if (len > PAGE_SIZE) {
+               page = mm_get_huge_zero_page(current->mm);
+               if (likely(page))
+                       huge_page = true;
+       }
+
+       if (!huge_page)
+               page = ZERO_PAGE(0);
+
+       fallback = ((len > PAGE_SIZE) && !huge_page);
+
That is pointless.
Bios can handle pages larger than PAGE_SIZE.

+       bio = iomap_dio_alloc_bio(iter, dio, BIO_MAX_VECS,
+                                 REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
Similar here. Just allocate space for a single folio.

         fscrypt_set_bio_crypt_ctx(bio, inode, pos >> inode->i_blkbits,
                                   GFP_KERNEL);
+
         bio->bi_iter.bi_sector = iomap_sector(&iter->iomap, pos);
         bio->bi_private = dio;
         bio->bi_end_io = iomap_dio_bio_end_io;

-       __bio_add_page(bio, page, len, 0);
+       if (!fallback) {
+               bio_add_folio_nofail(bio, page_folio(page), len, 0);
+       } else {
+               while (len) {
+                       unsigned int io_len =
+                               min_t(unsigned int, len, PAGE_SIZE);
+
+                       __bio_add_page(bio, page, io_len, 0);
+                       len -= io_len;
+               }
+       }
See above. Kill the 'fallback' case.

Cheers,

Hannes
--
Dr. Hannes Reinecke                Kernel Storage Architect
hare@xxxxxxx                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Ivo Totev, Andrew
Myers, Andrew McDonald, Martje Boudien Moerman




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