Re: [PATCH] mm, madvise: fix potential workingset node list_lru leaks

[Muchun Song <muchun.song@linux.dev>]

> On Dec 22, 2024, at 20:29, Kairui Song <ryncsn@gmail.com> wrote:
> 
> From: Kairui Song <kasong@tencent.com>
> 
> Since commit 5abc1e37afa0 ("mm: list_lru: allocate list_lru_one
> only when needed"), all list_lru users need to allocate the items using
> the new infrastructure that provides list_lru info for slab allocation,
> ensuring that the corresponding memcg list_lru is allocated before use.
> 
> For workingset shadow nodes (which are xa_node), users are converted to
> use the new infrastructure by commit 9bbdc0f32409 ("xarray: use
> kmem_cache_alloc_lru to allocate xa_node"). The xas->xa_lru will be
> set correctly for filemap users. However, there is a missing case:
> xa_node allocations caused by madvise(..., MADV_COLLAPSE).
> 
> madvise(..., MADV_COLLAPSE) will also read in the absent parts of file
> map, and there will be xa_nodes allocated for the caller's memcg
> (assuming it's not rootcg). However, these allocations won't trigger
> memcg list_lru allocation because the proper xas info was not set.
> 
> If nothing else has allocated other xa_nodes for that memcg to trigger
> list_lru creation, and memory pressure starts to evict file pages,
> workingset_update_node will try to add these xa_nodes to their
> corresponding memcg list_lru, and it does not exist (NULL). So they
> will be added to rootcg's list_lru instead.
> 
> This shouldn't be a significant issue in practice, but it is indeed
> unexpected behavior, and these xa_nodes will not be reclaimed
> effectively. And may lead to incorrect counting of the
> list_lru->nr_items counter.
> 
> This problem wasn't exposed until recent commit 28e98022b31ef
> ("mm/list_lru: simplify reparenting and initial allocation") added a
> sanity check: only dying memcg could have a NULL list_lru when
> list_lru_{add,del} is called. This problem triggered this WARNING.
> 
> So make madvise(..., MADV_COLLAPSE) also call xas_set_lru() to pass
> the list_lru which we may want to insert xa_node into later. And move
> mapping_set_update to mm/internal.h, and turn into a macro to avoid
> including extra headers in mm/internal.h.
> 
> Fixes: 9bbdc0f32409 ("xarray: use kmem_cache_alloc_lru to allocate xa_node")

Hi Kairui,

This should be 7d8faaf15545 introducing MADV_COLLAPSE, right? Because it is developed after 9bbdc0f32409.

Thanks.

> Reported-by: syzbot+38a0cbd267eff2d286ff@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/lkml/675d01e9.050a0220.37aaf.00be.GAE@google.com/
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
> mm/filemap.c    | 9 ---------
> mm/internal.h   | 6 ++++++
> mm/khugepaged.c | 3 +++
> 3 files changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index f61cf51c2238..33b60d448fca 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -124,15 +124,6 @@
>  *    ->private_lock        (zap_pte_range->block_dirty_folio)
>  */
> 
> -static void mapping_set_update(struct xa_state *xas,
> -        struct address_space *mapping)
> -{
> -    if (dax_mapping(mapping) || shmem_mapping(mapping))
> -        return;
> -    xas_set_update(xas, workingset_update_node);
> -    xas_set_lru(xas, &shadow_nodes);
> -}
> -
> static void page_cache_delete(struct address_space *mapping,
>                   struct folio *folio, void *shadow)
> {
> diff --git a/mm/internal.h b/mm/internal.h
> index cb8d8e8e3ffa..4e7a3a93d0a2 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1510,6 +1510,12 @@ static inline void shrinker_debugfs_remove(struct dentry *debugfs_entry,
> /* Only track the nodes of mappings with shadow entries */
> void workingset_update_node(struct xa_node *node);
> extern struct list_lru shadow_nodes;
> +#define mapping_set_update(xas, mapping) do {            \
> +    if (!dax_mapping(mapping) && !shmem_mapping(mapping)) {    \
> +        xas_set_update(xas, workingset_update_node);    \
> +        xas_set_lru(xas, &shadow_nodes);        \
> +    }                            \
> +} while (0)
> 
> /* mremap.c */
> unsigned long move_page_tables(struct vm_area_struct *vma,
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 6f8d46d107b4..653dbb1ff05c 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -19,6 +19,7 @@
> #include <linux/rcupdate_wait.h>
> #include <linux/swapops.h>
> #include <linux/shmem_fs.h>
> +#include <linux/dax.h>
> #include <linux/ksm.h>
> 
> #include <asm/tlb.h>
> @@ -1837,6 +1838,8 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>    if (result != SCAN_SUCCEED)
>        goto out;
> 
> +    mapping_set_update(&xas, mapping);
> +
>    __folio_set_locked(new_folio);
>    if (is_shmem)
>        __folio_set_swapbacked(new_folio);
> --
> 2.47.1
>