On Mon, 2021-06-14 at 13:25 +0100, David Howells wrote:
> Here's my take on Jeff's patch.
>
> David
> ---
> commit 2821dcb8a99b5da3b914cfc57ba6010635719b12
> Author: Jeff Layton <jlayton@xxxxxxxxxx>
> Date: Sun Jun 13 19:33:45 2021 -0400
>
> netfs: fix test for whether we can skip read when writing beyond EOF
>
> It's not sufficient to skip reading when the pos is beyond the EOF.
> There may be data at the head of the page that we need to fill in
> before the write.
>
> Add a new helper function that corrects and clarifies the logic of
> when we can skip reads, and have it only zero out the part of the page
> that won't have data copied in for the write.
>
> Finally, don't set the page Uptodate after zeroing. It's not up to date
> since the write data won't have been copied in yet.
>
> [DH made the following changes:
>
> - Prefixed the new function with "netfs_".
>
> - Don't call zero_user_segments() for a full-page write.
>
> - Altered the beyond-last-page check to avoid a DIV instruction and got
> rid of then-redundant zero-length file check.
>
> - Made afs handle a short copy (didn't matter before as the page was fully
> set up before the copy). Now it returns 0 from afs_write_end() if the
> page was not uptodate as commit b9de313cf05fe08fa59efaf19756ec5283af672a
> does for ceph.
>
> - Made afs handle len indicating a write that extended over the end of the
> page allocated.]
>
> Fixes: e1b1240c1ff5f ("netfs: Add write_begin helper")
> Reported-by: Andrew W Elble <aweits@xxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/340984.1623666185@xxxxxxxxxxxxxxxxxxxxxx/T/#m5e9b155310973bc71cf101d3dea6aced492bfd49
> ---
> fs/afs/write.c | 23 ++++++++++++++++------
> fs/netfs/read_helper.c | 50 ++++++++++++++++++++++++++++++++++++-------------
> 2 files changed, 54 insertions(+), 19 deletions(-)
>
> diff --git a/fs/afs/write.c b/fs/afs/write.c
> index 4e90e094d770..15fdc3f8a3ae 100644
> --- a/fs/afs/write.c
> +++ b/fs/afs/write.c
> @@ -28,7 +28,8 @@ int afs_set_page_dirty(struct page *page)
> }
>
> /*
> - * prepare to perform part of a write to a page
> + * Prepare to perform part of a write to a page. Note that len may extend
> + * beyond the end of the page.
> */
> int afs_write_begin(struct file *file, struct address_space *mapping,
> loff_t pos, unsigned len, unsigned flags,
> @@ -55,7 +56,8 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
> return ret;
>
> index = page->index;
> - from = pos - index * PAGE_SIZE;
> + from = offset_in_thp(page, pos);
> + len = min_t(size_t, len, thp_size(page) - from);
> to = from + len;
>
> try_again:
> @@ -106,7 +108,8 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
> }
>
> /*
> - * finalise part of a write to a page
> + * Finalise part of a write to a page. Note that len may extend beyond the end
> + * of the page.
> */
> int afs_write_end(struct file *file, struct address_space *mapping,
> loff_t pos, unsigned len, unsigned copied,
> @@ -115,13 +118,23 @@ int afs_write_end(struct file *file, struct address_space *mapping,
> struct afs_vnode *vnode = AFS_FS_I(file_inode(file));
> struct page *page = thp_head(subpage);
> unsigned long priv;
> - unsigned int f, from = pos & (thp_size(page) - 1);
> + unsigned int f, from = offset_in_thp(page, pos);
> unsigned int t, to = from + copied;
> + unsigned int l = min_t(size_t, len, thp_size(page) - from);
> loff_t i_size, write_end_pos;
>
> _enter("{%llx:%llu},{%lx}",
> vnode->fid.vid, vnode->fid.vnode, page->index);
>
> + if (!PageUptodate(page)) {
> + if (copied < l) {
> + copied = 0;
> + goto out;
> + }
> +
> + SetPageUptodate(page);
> + }
> +
> if (copied == 0)
> goto out;
>
> @@ -137,8 +150,6 @@ int afs_write_end(struct file *file, struct address_space *mapping,
> fscache_update_cookie(afs_vnode_cache(vnode), NULL, &write_end_pos);
> }
>
> - ASSERT(PageUptodate(page));
> -
> if (PagePrivate(page)) {
> priv = page_private(page);
> f = afs_page_dirty_from(page, priv);
AFS changes should probably go into a separate patch.
> diff --git a/fs/netfs/read_helper.c b/fs/netfs/read_helper.c
> index 725614625ed4..0c4e68ae8127 100644
> --- a/fs/netfs/read_helper.c
> +++ b/fs/netfs/read_helper.c
> @@ -1011,12 +1011,43 @@ int netfs_readpage(struct file *file,
> }
> EXPORT_SYMBOL(netfs_readpage);
>
> -static void netfs_clear_thp(struct page *page)
> +/**
> + * prep_noread_page - prep a page for writing without reading first
nit: fix function name here too?
> + * @page: page being prepared
> + * @pos: starting position for the write
> + * @len: length of write
> + *
> + * In some cases, write_begin doesn't need to read at all:
> + * - full page write
> + * - file is currently zero-length
> + * - write that lies in a page that is completely beyond EOF
> + * - write that covers the the page from start to EOF or beyond it
> + *
> + * If any of these criteria are met, then zero out the unwritten parts
> + * of the page and return true. Otherwise, return false.
> + */
> +static noinline bool netfs_prep_noread_page(struct page *page, loff_t pos, size_t len)
> {
> - unsigned int i;
> + struct inode *inode = page->mapping->host;
> + loff_t i_size = i_size_read(inode);
> + size_t offset = offset_in_thp(page, pos);
> +
> + /* Full page write */
> + if (offset == 0 && len >= thp_size(page))
> + return true;
> +
> + /* pos beyond last page in the file */
> + if (pos - offset >= i_size)
> + goto zero_out;
> +
> + /* Write that covers from the start of the page to EOF or beyond */
> + if (offset == 0 && (pos + len) >= i_size)
> + goto zero_out;
>
> - for (i = 0; i < thp_nr_pages(page); i++)
> - clear_highpage(page + i);
> + return false;
> +zero_out:
> + zero_user_segments(page, 0, offset, offset + len, thp_size(page));
> + return true;
> }
>
> /**
> @@ -1024,7 +1055,7 @@ static void netfs_clear_thp(struct page *page)
> * @file: The file to read from
> * @mapping: The mapping to read from
> * @pos: File position at which the write will begin
> - * @len: The length of the write in this page
> + * @len: The length of the write (may extend beyond the end of the page chosen)
> * @flags: AOP_* flags
> * @_page: Where to put the resultant page
> * @_fsdata: Place for the netfs to store a cookie
> @@ -1061,8 +1092,6 @@ int netfs_write_begin(struct file *file, struct address_space *mapping,
> struct inode *inode = file_inode(file);
> unsigned int debug_index = 0;
> pgoff_t index = pos >> PAGE_SHIFT;
> - int pos_in_page = pos & ~PAGE_MASK;
> - loff_t size;
> int ret;
>
> DEFINE_READAHEAD(ractl, file, NULL, mapping, index);
> @@ -1090,13 +1119,8 @@ int netfs_write_begin(struct file *file, struct address_space *mapping,
> * within the cache granule containing the EOF, in which case we need
> * to preload the granule.
> */
> - size = i_size_read(inode);
> if (!ops->is_cache_enabled(inode) &&
> - ((pos_in_page == 0 && len == thp_size(page)) ||
> - (pos >= size) ||
> - (pos_in_page == 0 && (pos + len) >= size))) {
> - netfs_clear_thp(page);
> - SetPageUptodate(page);
> + netfs_prep_noread_page(page, pos, len)) {
> netfs_stat(&netfs_n_rh_write_zskip);
> goto have_page_no_wait;
> }
>
The rest looks good to me.
--
Jeff Layton <jlayton@xxxxxxxxxx>
