On Thu, Sep 07, 2023 at 08:26:35AM +0530, Ritesh Harjani wrote:
> Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:
>
> > On Wed, Sep 06, 2023 at 01:38:23PM +0100, Matthew Wilcox wrote:
> >> > Is this code path a possibility, which can cause above logs?
> >> >
> >> > ptr = jbd2_alloc() -> kmem_cache_alloc()
> >> > <..>
> >> > new_folio = virt_to_folio(ptr)
> >> > new_offset = offset_in_folio(new_folio, ptr)
> >> >
> >> > And then I am still not sure what the problem really is?
> >> > Is it because at the time of checkpointing, the path is still not fully
> >> > converted to folio?
> >>
> >> Oh yikes! I didn't know that the allocation might come from kmalloc!
> >> Yes, slab might use high-order allocations. I'll have to look through
> >> this and figure out what the problem might be.
> >
> > I think the probable cause is bh_offset(). Before these patches, if
> > we allocated a buffer at offset 9kB into an order-2 slab, we'd fill in
> > b_page with the third page of the slab and calculate bh_offset as 1kB.
> > With these patches, we set b_page to the first page of the slab, and
> > bh_offset still comes back as 1kB so we read from / write to entirely
> > the wrong place.
> >
> > With this redefinition of bh_offset(), we calculate the offset relative
> > to the base page if it's a tail page, and relative to the folio if it's
> > a folio. Works out nicely ;-)
>
> Thanks Matthew for explaining the problem clearly.
>
>
> >
> > I have three other things I'm trying to debug right now, so this isn't
> > tested, but if you have time you might want to give it a run.
>
> sure, I gave it a try.
>
> >
> > diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> > index 6cb3e9af78c9..dc8fcdc40e95 100644
> > --- a/include/linux/buffer_head.h
> > +++ b/include/linux/buffer_head.h
> > @@ -173,7 +173,10 @@ static __always_inline int buffer_uptodate(const struct buffer_head *bh)
> > return test_bit_acquire(BH_Uptodate, &bh->b_state);
> > }
> >
> > -#define bh_offset(bh) ((unsigned long)(bh)->b_data & ~PAGE_MASK)
> > +static inline unsigned long bh_offset(struct buffer_head *bh)
> > +{
> > + return (unsigned long)(bh)->b_data & (page_size(bh->b_page) - 1);
> > +}
> >
> > /* If we *know* page->private refers to buffer_heads */
> > #define page_buffers(page) \
>
>
> I used "const" for bh to avoid warnings from fs/nilfs/alloc.c
Excellent. I didn't try compiling nilfs ;-)
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index 4ede47649a81..b61fa79cb7f5 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -171,7 +171,10 @@ static __always_inline int buffer_uptodate(const struct buffer_head *bh)
> return test_bit_acquire(BH_Uptodate, &bh->b_state);
> }
>
> -#define bh_offset(bh) ((unsigned long)(bh)->b_data & ~PAGE_MASK)
> +static inline unsigned long bh_offset(const struct buffer_head *bh)
> +{
> + return (unsigned long)(bh)->b_data & (page_size(bh->b_page) - 1);
> +}
>
> /* If we *know* page->private refers to buffer_heads */
> #define page_buffers(page) \
>
>
> But this change alone was still giving me failures. On looking into
> usage of b_data, I found we use offset_in_page() instead of bh_offset()
> in jbd2. So I added below changes in fs/jbd2 to replace offset_in_page()
> to bh_offset()...
>
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index 1073259902a6..0c25640714ac 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -304,7 +304,7 @@ static __u32 jbd2_checksum_data(__u32 crc32_sum, struct buffer_head *bh)
>
> addr = kmap_atomic(page);
> checksum = crc32_be(crc32_sum,
> - (void *)(addr + offset_in_page(bh->b_data)), bh->b_size);
> + (void *)(addr + bh_offset(bh)), bh->b_size);
> kunmap_atomic(addr);
Hm, that's not going to work on a highmem machine. It'll work on 64-bit!
Actually, no, it'll work on a highmem machine because slab doesn't
allocate from highmem. Still, it's a bit unclean. Let's go full folio
on this one:
diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 1073259902a6..8d6f934c3d95 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -298,14 +298,12 @@ static int journal_finish_inode_data_buffers(journal_t *journal,
static __u32 jbd2_checksum_data(__u32 crc32_sum, struct buffer_head *bh)
{
- struct page *page = bh->b_page;
char *addr;
__u32 checksum;
- addr = kmap_atomic(page);
- checksum = crc32_be(crc32_sum,
- (void *)(addr + offset_in_page(bh->b_data)), bh->b_size);
- kunmap_atomic(addr);
+ addr = kmap_local_folio(bh->b_folio, bh_offset(bh));
+ checksum = crc32_be(crc32_sum, addr, bh->b_size);
+ kunmap_local(addr);
return checksum;
}
@@ -322,7 +320,6 @@ static void jbd2_block_tag_csum_set(journal_t *j, journal_block_tag_t *tag,
struct buffer_head *bh, __u32 sequence)
{
journal_block_tag3_t *tag3 = (journal_block_tag3_t *)tag;
- struct page *page = bh->b_page;
__u8 *addr;
__u32 csum32;
__be32 seq;
@@ -331,11 +328,10 @@ static void jbd2_block_tag_csum_set(journal_t *j, journal_block_tag_t *tag,
return;
seq = cpu_to_be32(sequence);
- addr = kmap_atomic(page);
+ addr = kmap_local_folio(bh->b_folio, bh_offset(bh));
csum32 = jbd2_chksum(j, j->j_csum_seed, (__u8 *)&seq, sizeof(seq));
- csum32 = jbd2_chksum(j, csum32, addr + offset_in_page(bh->b_data),
- bh->b_size);
- kunmap_atomic(addr);
+ csum32 = jbd2_chksum(j, csum32, addr, bh->b_size);
+ kunmap_local(addr);
if (jbd2_has_feature_csum3(j))
tag3->t_checksum = cpu_to_be32(csum32);
diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 4d1fda1f7143..5f08b5fd105a 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -935,19 +935,15 @@ static void warn_dirty_buffer(struct buffer_head *bh)
/* Call t_frozen trigger and copy buffer data into jh->b_frozen_data. */
static void jbd2_freeze_jh_data(struct journal_head *jh)
{
- struct page *page;
- int offset;
char *source;
struct buffer_head *bh = jh2bh(jh);
J_EXPECT_JH(jh, buffer_uptodate(bh), "Possible IO failure.\n");
- page = bh->b_page;
- offset = offset_in_page(bh->b_data);
- source = kmap_atomic(page);
+ source = kmap_local_folio(bh->b_folio, bh_offset(bh));
/* Fire data frozen trigger just before we copy the data */
- jbd2_buffer_frozen_trigger(jh, source + offset, jh->b_triggers);
- memcpy(jh->b_frozen_data, source + offset, bh->b_size);
- kunmap_atomic(source);
+ jbd2_buffer_frozen_trigger(jh, source, jh->b_triggers);
+ memcpy(jh->b_frozen_data, source, bh->b_size);
+ kunmap_local(source);
/*
* Now that the frozen data is saved off, we need to store any matching
(I've been thinking about adding a kmap_local_bh(bh))
> ext4/1k: 15 tests, 1 failures, 1709 seconds
> generic/455 Pass 43s
> generic/475 Pass 128s
> generic/482 Pass 183s
> generic/455 Pass 43s
> generic/475 Pass 134s
> generic/482 Pass 191s
> generic/455 Pass 41s
> generic/475 Pass 139s
> generic/482 Pass 135s
> generic/455 Pass 46s
> generic/475 Pass 132s
> generic/482 Pass 146s
> generic/455 Pass 47s
> generic/475 Failed 145s
> generic/482 Pass 156s
> Totals: 15 tests, 0 skipped, 1 failures, 0 errors, 1709s
>
> I guess the above failure (generic/475) could be due to it's flakey
> behaviour which Ted was mentioning.
>
>
> Now, while we are at it, I think we should also make change to reiserfs from
> offset_in_page() to bh_offset()
>
> diff --git a/fs/reiserfs/journal.c b/fs/reiserfs/journal.c
> index 015bfe4e4524..23411ec163d4 100644
> --- a/fs/reiserfs/journal.c
> +++ b/fs/reiserfs/journal.c
> @@ -4217,7 +4217,7 @@ static int do_journal_end(struct reiserfs_transaction_handle *th, int flags)
> page = cn->bh->b_page;
> addr = kmap(page);
> memcpy(tmp_bh->b_data,
> - addr + offset_in_page(cn->bh->b_data),
> + addr + bh_offset(cn->bh),
> cn->bh->b_size);
> kunmap(page);
This one should probably be:
- addr = kmap(page);
- memcpy(tmp_bh->b_data,
- addr + offset_in_page(cn->bh->b_data),
- cn->bh->b_size);
- kunmap(page);
+ memcpy_from_folio(tmp_bh->b_data, cn->bh->b_folio,
+ bh_offset(cn->bh), cn->bh->b_size);
> I will also run "auto" group with ext4/1k with all of above change. Will
> update the results once it is done.
Appreciate it! I don't think you'll see a significant difference with
the patches above; you've nailed the actual problems and I'm just
using slighlty nicer APIs.
