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Date: Thu, 15 Mar 2007 12:44:37 +0300
From: Dmitriy Monakhov <dmonakhov@...ru>
To: Nick Piggin <npiggin@...e.de>
Cc: Linux Filesystems <linux-fsdevel@...r.kernel.org>,
Linux Kernel <linux-kernel@...r.kernel.org>,
Christoph Hellwig <hch@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Mark Fasheh <mark.fasheh@...cle.com>
Subject: Re: [patch 2/5] fs: introduce new aops and infrastructure
Nick Piggin <npiggin@...e.de> writes:
> Index: linux-2.6/fs/splice.c
> ===================================================================
> --- linux-2.6.orig/fs/splice.c
> +++ linux-2.6/fs/splice.c
> @@ -559,7 +559,7 @@ static int pipe_to_file(struct pipe_inod
> struct address_space *mapping = file->f_mapping;
> unsigned int offset, this_len;
> struct page *page;
> - pgoff_t index;
> + void *fsdata;
> int ret;
>
> /*
> @@ -569,13 +569,13 @@ static int pipe_to_file(struct pipe_inod
> if (unlikely(ret))
> return ret;
>
> - index = sd->pos >> PAGE_CACHE_SHIFT;
> offset = sd->pos & ~PAGE_CACHE_MASK;
>
> this_len = sd->len;
> if (this_len + offset > PAGE_CACHE_SIZE)
> this_len = PAGE_CACHE_SIZE - offset;
>
> +#if 0
> /*
> * Reuse buf page, if SPLICE_F_MOVE is set and we are doing a full
> * page.
> @@ -587,86 +587,11 @@ static int pipe_to_file(struct pipe_inod
> * locked on successful return.
> */
> if (buf->ops->steal(pipe, buf))
> - goto find_page;
> +#endif
One more note. It's looks like you just disabled all fancy zero copy logic.
Off corse this is just rfc patchset.
But i think where is fundamental problem with it:
Previous logic was following:
1)splice code responsible for: stealing(if possible) and loking the page
2)prepare_write() code responsible for: do fs speciffic stuff
But with new write_begin() logic all steps (grubbing, locking, preparing)
happened internaly inside write_begin() witch doesn't even know about what
kind of data will be copied between write_begin/write_end.
So fancy zero copy logic is impossible :(
I think this can be solved somehow, but i dont know yet, how can this be done
without implementing it inside begin_write().
>
> - page = buf->page;
> - if (add_to_page_cache(page, mapping, index, GFP_KERNEL)) {
> - unlock_page(page);
> - goto find_page;
> - }
> -
> - page_cache_get(page);
> -
> - if (!(buf->flags & PIPE_BUF_FLAG_LRU))
> - lru_cache_add(page);
> - } else {
> -find_page:
> - page = find_lock_page(mapping, index);
> - if (!page) {
> - ret = -ENOMEM;
> - page = page_cache_alloc_cold(mapping);
> - if (unlikely(!page))
> - goto out_ret;
> -
> - /*
> - * This will also lock the page
> - */
> - ret = add_to_page_cache_lru(page, mapping, index,
> - GFP_KERNEL);
> - if (unlikely(ret))
> - goto out;
> - }
> -
> - /*
> - * We get here with the page locked. If the page is also
> - * uptodate, we don't need to do more. If it isn't, we
> - * may need to bring it in if we are not going to overwrite
> - * the full page.
> - */
> - if (!PageUptodate(page)) {
> - if (this_len < PAGE_CACHE_SIZE) {
> - ret = mapping->a_ops->readpage(file, page);
> - if (unlikely(ret))
> - goto out;
> -
> - lock_page(page);
> -
> - if (!PageUptodate(page)) {
> - /*
> - * Page got invalidated, repeat.
> - */
> - if (!page->mapping) {
> - unlock_page(page);
> - page_cache_release(page);
> - goto find_page;
> - }
> - ret = -EIO;
> - goto out;
> - }
> - } else
> - SetPageUptodate(page);
> - }
> - }
> -
> - ret = mapping->a_ops->prepare_write(file, page, offset, offset+this_len);
> - if (unlikely(ret)) {
> - loff_t isize = i_size_read(mapping->host);
> -
> - if (ret != AOP_TRUNCATED_PAGE)
> - unlock_page(page);
> - page_cache_release(page);
> - if (ret == AOP_TRUNCATED_PAGE)
> - goto find_page;
> -
> - /*
> - * prepare_write() may have instantiated a few blocks
> - * outside i_size. Trim these off again.
> - */
> - if (sd->pos + this_len > isize)
> - vmtruncate(mapping->host, isize);
> -
> - goto out_ret;
> - }
> + ret = pagecache_write_begin(file, mapping, sd->pos, sd->len, 0, &page, &fsdata);
> + if (unlikely(ret))
> + goto out;
>
> if (buf->page != page) {
> /*
> @@ -676,28 +601,13 @@ find_page:
> char *dst = kmap_atomic(page, KM_USER1);
>
> memcpy(dst + offset, src + buf->offset, this_len);
> - flush_dcache_page(page);
> kunmap_atomic(dst, KM_USER1);
> buf->ops->unmap(pipe, buf, src);
> }
>
> - ret = mapping->a_ops->commit_write(file, page, offset, offset+this_len);
> - if (!ret) {
> - /*
> - * Return the number of bytes written and mark page as
> - * accessed, we are now done!
> - */
> - ret = this_len;
> - mark_page_accessed(page);
> - balance_dirty_pages_ratelimited(mapping);
> - } else if (ret == AOP_TRUNCATED_PAGE) {
> - page_cache_release(page);
> - goto find_page;
> - }
> + ret = pagecache_write_end(file, mapping, sd->pos, sd->len, sd->len, page, fsdata);
> +
> out:
> - page_cache_release(page);
> - unlock_page(page);
> -out_ret:
> return ret;
> }
>
-
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