Re: [PATCH 1/7] xfs: create a big array data structure

Matthew Wilcox
2023-07-28T03:11:09Z

On Thu, Jul 27, 2023 at 03:25:35PM -0700, Darrick J. Wong wrote:
> diff --git a/fs/xfs/scrub/trace.h b/fs/xfs/scrub/trace.h
> index 7418d6c60056a..0b9e781840f37 100644
> --- a/fs/xfs/scrub/trace.h
> +++ b/fs/xfs/scrub/trace.h
> @@ -16,6 +16,9 @@
> #include <linux/tracepoint.h>
> #include "xfs_bit.h"
>
> +struct xfile;
> +struct xfarray;

You dereference both a struct xfile and a struct xfarray. Seems like
you don't need these declarations?

> +/* Compute array index given an xfile offset. */
> +static xfarray_idx_t
> +xfarray_idx(
> + struct xfarray *array,
> + loff_t pos)
> +{
> + if (array->obj_size_log >= 0)
> + return (xfarray_idx_t)pos >> array->obj_size_log;
> +
> + return div_u64((xfarray_idx_t)pos, array->obj_size);

If xfarray_idx_t is smaller than an loff_t, this will truncate pos,
which isn't what you want.

> +/* Compute xfile offset of array element. */
> +static inline loff_t xfarray_pos(struct xfarray *array, xfarray_idx_t idx)
> +{
> + if (array->obj_size_log >= 0)
> + return idx << array->obj_size_log;
> +
> + return idx * array->obj_size;

Likewise, you need to promote idx to loff_t before shifting/multiplying.

> +static inline bool
> +xfarray_is_unset(
> + struct xfarray *array,
> + loff_t pos)
> +{
> + void *temp = xfarray_scratch(array);
> + int error;
> +
> + if (array->unset_slots == 0)
> + return false;
> +
> + error = xfile_obj_load(array->xfile, temp, array->obj_size, pos);
> + if (!error && xfarray_element_is_null(array, temp))
> + return true;
> +
> + return false;

Wouldn't this be clearer as:

return !error && xfarray_element_is_null(array, temp);

> +int
> +xfarray_store_anywhere(
> + struct xfarray *array,
> + const void *ptr)
> +{
> + void *temp = xfarray_scratch(array);
> + loff_t endpos = xfarray_pos(array, array->nr);
> + loff_t pos;
> + int error;
> +
> + /* Find an unset slot to put it in. */
> + for (pos = 0;
> + pos < endpos && array->unset_slots > 0;
> + pos += array->obj_size) {
> + error = xfile_obj_load(array->xfile, temp, array->obj_size,
> + pos);
> + if (error || !xfarray_element_is_null(array, temp))
> + continue;
> +
> + error = xfile_obj_store(array->xfile, ptr, array->obj_size,
> + pos);
> + if (error)
> + return error;
> +
> + array->unset_slots--;
> + return 0;
> + }

... how often is this called? This seems like it might be slow.

> + /*
> + * Call SEEK_DATA on the last byte in the record we're about to read.
> + * If the record ends at (or crosses) the end of a page then we know
> + * that the first byte of the record is backed by pages and don't need
> + * to query it. If instead the record begins at the start of the page
> + * then we know that querying the last byte is just as good as querying
> + * the first byte, since records cannot be larger than a page.
> + *
> + * If the call returns the same file offset, we know this record is
> + * backed by real pages. We do not need to move the cursor.
> + */

Clever.

> +ssize_t
> +xfile_pread(
> + struct xfile *xf,
> + void *buf,
> + size_t count,
> + loff_t pos)
> +{
> + struct inode *inode = file_inode(xf->file);
> + struct address_space *mapping = inode->i_mapping;
> + struct page *page = NULL;
> + ssize_t read = 0;
> + unsigned int pflags;
> + int error = 0;
> +
> + if (count > MAX_RW_COUNT)
> + return -E2BIG;
> + if (inode->i_sb->s_maxbytes - pos < count)
> + return -EFBIG;
> +
> + trace_xfile_pread(xf, pos, count);
> +
> + pflags = memalloc_nofs_save();

Should we be calling this here, or should this be done by the caller?
Presumably it's the current caller that can't stand reclaim starting?

> + while (count > 0) {
> + void *p, *kaddr;
> + unsigned int len;
> +
> + len = min_t(ssize_t, count, PAGE_SIZE - offset_in_page(pos));
> +
> + /*
> + * In-kernel reads of a shmem file cause it to allocate a page
> + * if the mapping shows a hole. Therefore, if we hit ENOMEM
> + * we can continue by zeroing the caller's buffer.
> + */
> + page = shmem_read_mapping_page_gfp(mapping, pos >> PAGE_SHIFT,
> + __GFP_NOWARN);

I'm kind of hoping to transition to shmem_read_folio_gfp(), but that
doesn't have to happen before this gets merged.

> +ssize_t
> +xfile_pwrite(
> + struct xfile *xf,
> + const void *buf,
> + size_t count,
> + loff_t pos)
> +{
> + struct inode *inode = file_inode(xf->file);
> + struct address_space *mapping = inode->i_mapping;

I wonder if this shoudn't be xf->file->f_mapping?