Re: [PATCH 1/2] mm/shmem: add shmem_insert_folio()

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

Hi David,

Thanks for having a look.

On Tue, 2026-05-12 at 13:07 +0200, David Hildenbrand (Arm) wrote:
> 
> >  
> > +/**
> > + * undo_compound_page() - Reverse the effect of
> > prep_compound_page().
> > + * @page: The head page of a compound page to demote.
> > + *
> > + * Returns the pages to non-compound state as if
> > prep_compound_page()
> > + * had never been called.  split_page() must NOT have been called
> > on
> > + * the compound page; tail refcounts must be 0.  The caller must
> > ensure
> > + * no other users hold references to the compound page.
> > + */
> > +void undo_compound_page(struct page *page)
> > +{
> > +	unsigned int i, nr = 1U << compound_order(page);
> > +
> > +	page[1].flags.f &= ~PAGE_FLAGS_SECOND;
> > +	for (i = 1; i < nr; i++) {
> > +		page[i].mapping = NULL;
> > +		clear_compound_head(&page[i]);
> > +	}
> > +	ClearPageHead(page);
> > +}
> > +
> >  static inline void set_buddy_order(struct page *page, unsigned int
> > order)
> >  {
> >  	set_page_private(page, order);
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 3b5dc21b323c..45e80a74f77c 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -937,6 +937,111 @@ int shmem_add_to_page_cache(struct folio
> > *folio,
> >  	return 0;
> >  }
> >  
> > +/**
> > + * shmem_insert_folio() - Insert an isolated folio into a shmem
> > file.
> > + * @file: The shmem file created with shmem_file_setup().
> > + * @folio: The folio to insert. Must be isolated (not on LRU),
> > unlocked,
> > + *         have exactly one reference (the caller's), have no
> > page-table
> > + *         mappings, and have folio->mapping == NULL.
> > + * @order: The allocation order of @folio.  If @order > 0 and
> > @folio is
> > + *         not already a large (compound) folio, it will be
> > promoted to a
> > + *         compound folio of this order inside this function. 
> > This requires
> > + *         the standard post-alloc state: head refcount == 1, tail
> > + *         refcounts == 0 (i.e. split_page() must NOT have been
> > called).
> > + *         On failure the promotion is reversed and the folio is
> > returned
> > + *         to its original non-compound state.
> > + * @index: Page-cache index at which to insert. Must be aligned to
> > + *         (1 << @order) and within the file's size.
> > + * @writeback: If true, attempt immediate writeback to swap after
> > insertion.
> > + *             Best-effort; failure is silently ignored.
> > + * @folio_gfp: The GFP flags to use for memory-cgroup charging.
> > + *
> > + * The folio is inserted zero-copy into the shmem page cache and
> > placed on
> > + * the anon LRU, where it participates in normal kernel reclaim
> > (written to
> > + * swap under memory pressure).  Any previous content at @index is
> > discarded.
> > + * On success the caller should release their reference with
> > folio_put() and
> > + * track the (@file, @index) pair for later recovery via
> > shmem_read_folio()
> > + * and release via shmem_truncate_range().
> > + *
> > + * Return: 0 on success.  On failure the folio is returned to its
> > original
> > + * state and the caller retains ownership.
> > + */
> > +int shmem_insert_folio(struct file *file, struct folio *folio,
> > unsigned int order,
> > +		       pgoff_t index, bool writeback, gfp_t
> > folio_gfp)
> > +{
> > +	struct address_space *mapping = file->f_mapping;
> > +	struct inode *inode = mapping->host;
> > +	bool promoted;
> > +	long nr_pages;
> > +	int ret;
> > +
> > +	promoted = order > 0 && !folio_test_large(folio);
> > +	if (promoted)
> > +		prep_compound_page(&folio->page, order);
> > +	nr_pages = folio_nr_pages(folio);
> > +
> > +	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
> > +	VM_BUG_ON_FOLIO(folio_mapped(folio), folio);
> > +	VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
> > +	VM_BUG_ON_FOLIO(folio->mapping, folio);
> > +	VM_BUG_ON(index != round_down(index, nr_pages));
> 
> No new VM_BUG_ON_FOLIO etc.

OK, can eliminate those. Is VM_WARN_ON_FOLIO() preferred,
or any other type of assert?

> 
> But in general, pushing in random allocated pages into shmem,
> converting them to
> folios is not something I particularly enjoy seeing.
> 

OK, let me understand the concern. The pages are allocated as multi-
page folios using alloc_pages(gfp, order), but typically not promoted
to compound pages, until inserted here. Is it that promotion that is of
concern or inserting pages of unknown origin into shmem? Anything we
can do to alleviate that concern?

Given the problem statement in the cover-letter, would there be a
better direction to take here? We could, for example, bypass shmem and
insert the folios directly into the swap-cache, (although there is an
issue with the swap-cache when the number of swap_entries are close to
being depleted).

https://patchwork.freedesktop.org/series/165518/

Thanks,
Thomas