Re: [patch 6/6] mm: fix pagecache write deadlocks

Andrew Morton
2006-10-13T22:14:57Z

On Fri, 13 Oct 2006 18:44:52 +0200 (CEST)
Nick Piggin <npiggin@suse.de> wrote:

> From: Andrew Morton <akpm@osdl.org> and Nick Piggin <npiggin@suse.de>
>
> The idea is to modify the core write() code so that it won't take a pagefault
> while holding a lock on the pagecache page. There are a number of different
> deadlocks possible if we try to do such a thing:
>
> 1. generic_buffered_write
> 2. lock_page
> 3. prepare_write
> 4. unlock_page+vmtruncate
> 5. copy_from_user
> 6. mmap_sem(r)
> 7. handle_mm_fault
> 8. lock_page (filemap_nopage)
> 9. commit_write
> 1. unlock_page
>
> b. sys_munmap / sys_mlock / others
> c. mmap_sem(w)
> d. make_pages_present
> e. get_user_pages
> f. handle_mm_fault
> g. lock_page (filemap_nopage)
>
> 2,8 - recursive deadlock if page is same
> 2,8;2,7 - ABBA deadlock is page is different
> 2,6;c,g - ABBA deadlock if page is same
>
> - Instead of copy_from_user(), use inc_preempt_count() and
> copy_from_user_inatomic().
>
> - If the copy_from_user_inatomic() hits a pagefault, it'll return a short
> copy.
>
> - if the page was not uptodate, we cannot commit the write, because the
> uncopied bit could have uninitialised data. Commit zero length copy,
> which should do the right thing (ie. not set the page uptodate).
>
> - if the page was uptodate, commit the copied portion so we make some
> progress.
>
> - unlock_page()
>
> - go back and try to fault the page in again, redo the lock_page,
> prepare_write, copy_from_user_inatomic(), etc.
>
> - Now we have a non uptodate page, and we keep faulting on a 2nd or later
> iovec, this gives a deadlock, because fault_in_pages readable only faults
> in the first iovec. To fix this situation, if we fault on a !uptodate page,
> make the next iteration only attempt to copy a single iovec.
>
> - This also showed up a number of buggy prepare_write / commit_write
> implementations that were setting the page uptodate in the prepare_write
> side: bad! this allows uninitialised data to be read. Fix these.

Well. It's non-buggy under the current protocol because the page remains
locked throughout. This patch would make these ->prepare_write()
implementations buggy.

> +#ifdef CONFIG_DEBUG_VM
> + fault_in_pages_readable(buf, bytes);
> +#endif

I'll need to remember to take that out later on. Or reorder the patches, I
guess.

> int simple_commit_write(struct file *file, struct page *page,
> - unsigned offset, unsigned to)
> + unsigned from, unsigned to)
> {
> - struct inode *inode = page->mapping->host;
> - loff_t pos = ((loff_t)page->index << PAGE_CACHE_SHIFT) + to;
> -
> - /*
> - * No need to use i_size_read() here, the i_size
> - * cannot change under us because we hold the i_mutex.
> - */
> - if (pos > inode->i_size)
> - i_size_write(inode, pos);
> - set_page_dirty(page);
> + if (to > from) {
> + struct inode *inode = page->mapping->host;
> + loff_t pos = ((loff_t)page->index << PAGE_CACHE_SHIFT) + to;
> +
> + if (to - from == PAGE_CACHE_SIZE)
> + SetPageUptodate(page);

This SetPageUptodate() can go away?

> @@ -2317,17 +2320,6 @@ int nobh_prepare_write(struct page *page
>
> if (is_mapped_to_disk)
> SetPageMappedToDisk(page);
> - SetPageUptodate(page);
> -
> - /*
> - * Setting the page dirty here isn't necessary for the prepare_write
> - * function - commit_write will do that. But if/when this function is
> - * used within the pagefault handler to ensure that all mmapped pages
> - * have backing space in the filesystem, we will need to dirty the page
> - * if its contents were altered.
> - */
> - if (dirtied_it)
> - set_page_dirty(page);
>
> return 0;

Local variable `dirtied_it' can go away now.

Or can it? We've modified the page's contents. If the copy_from_user gets
a fault and we do a zero-length ->commit_write(), nobody ends up dirtying
the page.

> @@ -2450,6 +2436,7 @@ int nobh_truncate_page(struct address_sp
> memset(kaddr + offset, 0, PAGE_CACHE_SIZE - offset);
> flush_dcache_page(page);
> kunmap_atomic(kaddr, KM_USER0);
> + SetPageUptodate(page);
> set_page_dirty(page);
> }
> unlock_page(page);

I've already forgotten why this was added. Comment, please ;)