Re: [PATCH] squashfs: enable __GFP_FS in ->readpage to prevent hang in mem alloc

[Hou Tao <houtao1@huawei.com>]

ping ?

On 2018/12/4 10:08, Hou Tao wrote:
> There is no need to disable __GFP_FS in ->readpage:
> * It's a read-only fs, so there will be no dirty/writeback page and
>   there will be no deadlock against the caller's locked page
> * It just allocates one page, so compaction will not be invoked
> * It doesn't take any inode lock, so the reclamation of inode will be fine
> 
> And no __GFP_FS may lead to hang in __alloc_pages_slowpath() if a
> squashfs page fault occurs in the context of a memory hogger, because
> the hogger will not be killed due to the logic in __alloc_pages_may_oom().
> 
> Signed-off-by: Hou Tao <houtao1@huawei.com>
> ---
>  fs/squashfs/file.c          |  3 ++-
>  fs/squashfs/file_direct.c   |  4 +++-
>  fs/squashfs/squashfs_fs_f.h | 25 +++++++++++++++++++++++++
>  3 files changed, 30 insertions(+), 2 deletions(-)
>  create mode 100644 fs/squashfs/squashfs_fs_f.h
> 
> diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
> index f1c1430ae721..8603dda4a719 100644
> --- a/fs/squashfs/file.c
> +++ b/fs/squashfs/file.c
> @@ -51,6 +51,7 @@
>  #include "squashfs_fs.h"
>  #include "squashfs_fs_sb.h"
>  #include "squashfs_fs_i.h"
> +#include "squashfs_fs_f.h"
>  #include "squashfs.h"
>  
>  /*
> @@ -414,7 +415,7 @@ void squashfs_copy_cache(struct page *page, struct squashfs_cache_entry *buffer,
>  		TRACE("bytes %d, i %d, available_bytes %d\n", bytes, i, avail);
>  
>  		push_page = (i == page->index) ? page :
> -			grab_cache_page_nowait(page->mapping, i);
> +			squashfs_grab_cache_page_nowait(page->mapping, i);
>  
>  		if (!push_page)
>  			continue;
> diff --git a/fs/squashfs/file_direct.c b/fs/squashfs/file_direct.c
> index 80db1b86a27c..a0fdd6215348 100644
> --- a/fs/squashfs/file_direct.c
> +++ b/fs/squashfs/file_direct.c
> @@ -17,6 +17,7 @@
>  #include "squashfs_fs.h"
>  #include "squashfs_fs_sb.h"
>  #include "squashfs_fs_i.h"
> +#include "squashfs_fs_f.h"
>  #include "squashfs.h"
>  #include "page_actor.h"
>  
> @@ -60,7 +61,8 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
>  	/* Try to grab all the pages covered by the Squashfs block */
>  	for (missing_pages = 0, i = 0, n = start_index; i < pages; i++, n++) {
>  		page[i] = (n == target_page->index) ? target_page :
> -			grab_cache_page_nowait(target_page->mapping, n);
> +			squashfs_grab_cache_page_nowait(
> +					target_page->mapping, n);
>  
>  		if (page[i] == NULL) {
>  			missing_pages++;
> diff --git a/fs/squashfs/squashfs_fs_f.h b/fs/squashfs/squashfs_fs_f.h
> new file mode 100644
> index 000000000000..fc5fb7aeb27d
> --- /dev/null
> +++ b/fs/squashfs/squashfs_fs_f.h
> @@ -0,0 +1,25 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef SQUASHFS_FS_F
> +#define SQUASHFS_FS_F
> +
> +/*
> + * No need to use FGP_NOFS here:
> + * 1. It's a read-only fs, so there will be no dirty/writeback page and
> + *    there will be no deadlock against the caller's locked page.
> + * 2. It just allocates one page, so compaction will not be invoked.
> + * 3. It doesn't take any inode lock, so the reclamation of inode
> + *    will be fine.
> + *
> + * And GFP_NOFS may lead to infinite loop in __alloc_pages_slowpath() if a
> + * squashfs page fault occurs in the context of a memory hogger, because
> + * the hogger will not be killed due to the logic in __alloc_pages_may_oom().
> + */
> +static inline struct page *
> +squashfs_grab_cache_page_nowait(struct address_space *mapping, pgoff_t index)
> +{
> +	return pagecache_get_page(mapping, index,
> +			FGP_LOCK|FGP_CREAT|FGP_NOWAIT,
> +			mapping_gfp_mask(mapping));
> +}
> +#endif
> +
>