[PATCH v14 2/8] drm/ttm: Provide a shmem backup implementation

[Christian König]

Am 15.11.24 um 16:01 schrieb Thomas Hellström:
> Provide a standalone shmem backup implementation.
> Given the ttm_backup interface, this could
> later on be extended to providing other backup
> implementation than shmem, with one use-case being
> GPU swapout to a user-provided fd.
>
> v5:
> - Fix a UAF. (kernel test robot, Dan Carptenter)
> v6:
> - Rename ttm_backup_shmem_copy_page() function argument
>    (Matthew Brost)
> - Add some missing documentation
> v8:
> - Use folio_file_page to get to the page we want to writeback
>    instead of using the first page of the folio.
> v13:
> - Remove the base class abstraction (Christian König)
> - Include ttm_backup_bytes_avail().
> v14:
> - Fix kerneldoc for ttm_backup_bytes_avail() (0-day)
> - Work around casting of __randomize_layout struct pointer (0-day)
>
> Cc: Christian König <christian.koenig at amd.com>
> Cc: Somalapuram Amaranath <Amaranath.Somalapuram at amd.com>
> Cc: Matthew Brost <matthew.brost at intel.com>
> Cc: <dri-devel at lists.freedesktop.org>
> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Reviewed-by: Matthew Brost <matthew.brost at intel.com> #v13
> ---
>   drivers/gpu/drm/ttm/Makefile     |   2 +-
>   drivers/gpu/drm/ttm/ttm_backup.c | 204 +++++++++++++++++++++++++++++++
>   include/drm/ttm/ttm_backup.h     |  74 +++++++++++
>   3 files changed, 279 insertions(+), 1 deletion(-)
>   create mode 100644 drivers/gpu/drm/ttm/ttm_backup.c
>   create mode 100644 include/drm/ttm/ttm_backup.h
>
> diff --git a/drivers/gpu/drm/ttm/Makefile b/drivers/gpu/drm/ttm/Makefile
> index dad298127226..40d07a35293a 100644
> --- a/drivers/gpu/drm/ttm/Makefile
> +++ b/drivers/gpu/drm/ttm/Makefile
> @@ -4,7 +4,7 @@
>   
>   ttm-y := ttm_tt.o ttm_bo.o ttm_bo_util.o ttm_bo_vm.o ttm_module.o \
>   	ttm_execbuf_util.o ttm_range_manager.o ttm_resource.o ttm_pool.o \
> -	ttm_device.o ttm_sys_manager.o
> +	ttm_device.o ttm_sys_manager.o ttm_backup.o
>   ttm-$(CONFIG_AGP) += ttm_agp_backend.o
>   
>   obj-$(CONFIG_DRM_TTM) += ttm.o
> diff --git a/drivers/gpu/drm/ttm/ttm_backup.c b/drivers/gpu/drm/ttm/ttm_backup.c
> new file mode 100644
> index 000000000000..bf16bb0c594e
> --- /dev/null
> +++ b/drivers/gpu/drm/ttm/ttm_backup.c
> @@ -0,0 +1,204 @@
> +// SPDX-License-Identifier: MIT
> +/*
> + * Copyright © 2024 Intel Corporation
> + */
> +
> +#include <drm/ttm/ttm_backup.h>
> +#include <linux/page-flags.h>
> +#include <linux/swap.h>
> +
> +/*
> + * Casting from randomized struct file * to struct ttm_backup * is fine since
> + * struct ttm_backup is never defined nor dereferenced.
> + */
> +static struct file *ttm_backup_to_file(struct ttm_backup *backup)

Do I get it right that struct ttm_backup is never really defined? What 
purpose does that have?

> +{
> +	return (void *)backup;
> +}
> +
> +static struct ttm_backup *ttm_file_to_backup(struct file *file)
> +{
> +	return (void *)file;
> +}
> +
> +/*
> + * Need to map shmem indices to handle since a handle value
> + * of 0 means error, following the swp_entry_t convention.
> + */
> +static unsigned long ttm_backup_shmem_idx_to_handle(pgoff_t idx)
> +{
> +	return (unsigned long)idx + 1;
> +}
> +
> +static pgoff_t ttm_backup_handle_to_shmem_idx(pgoff_t handle)
> +{
> +	return handle - 1;
> +}
> +
> +/**
> + * ttm_backup_drop() - release memory associated with a handle
> + * @backup: The struct backup pointer used to obtain the handle
> + * @handle: The handle obtained from the @backup_page function.
> + */
> +void ttm_backup_drop(struct ttm_backup *backup, pgoff_t handle)
> +{
> +	loff_t start = ttm_backup_handle_to_shmem_idx(handle);
> +
> +	start <<= PAGE_SHIFT;
> +	shmem_truncate_range(file_inode(ttm_backup_to_file(backup)), start,
> +			     start + PAGE_SIZE - 1);
> +}
> +
> +/**
> + * ttm_backup_copy_page() - Copy the contents of a previously backed
> + * up page
> + * @backup: The struct backup pointer used to back up the page.
> + * @dst: The struct page to copy into.
> + * @handle: The handle returned when the page was backed up.
> + * @intr: Try to perform waits interruptable or at least killable.
> + *
> + * Return: 0 on success, Negative error code on failure, notably
> + * -EINTR if @intr was set to true and a signal is pending.
> + */
> +int ttm_backup_copy_page(struct ttm_backup *backup, struct page *dst,
> +			 pgoff_t handle, bool intr)
> +{
> +	struct file *filp = ttm_backup_to_file(backup);
> +	struct address_space *mapping = filp->f_mapping;
> +	struct folio *from_folio;
> +	pgoff_t idx = ttm_backup_handle_to_shmem_idx(handle);
> +
> +	from_folio = shmem_read_folio(mapping, idx);
> +	if (IS_ERR(from_folio))
> +		return PTR_ERR(from_folio);
> +
> +	copy_highpage(dst, folio_file_page(from_folio, idx));
> +	folio_put(from_folio);
> +
> +	return 0;
> +}
> +
> +/**
> + * ttm_backup_backup_page() - Backup a page
> + * @backup: The struct backup pointer to use.
> + * @page: The page to back up.
> + * @writeback: Whether to perform immediate writeback of the page.
> + * This may have performance implications.
> + * @idx: A unique integer for each page and each struct backup.
> + * This allows the backup implementation to avoid managing
> + * its address space separately.
> + * @page_gfp: The gfp value used when the page was allocated.
> + * This is used for accounting purposes.
> + * @alloc_gfp: The gpf to be used when allocating memory.

Typo: gfp instead of gpf.

> + *
> + * Context: If called from reclaim context, the caller needs to
> + * assert that the shrinker gfp has __GFP_FS set, to avoid
> + * deadlocking on lock_page(). If @writeback is set to true and
> + * called from reclaim context, the caller also needs to assert
> + * that the shrinker gfp has __GFP_IO set, since without it,
> + * we're not allowed to start backup IO.
> + *
> + * Return: A handle on success. 0 on failure.
> + * (This is following the swp_entry_t convention).
> + *
> + * Note: This function could be extended to back up a folio and
> + * implementations would then split the folio internally if needed.
> + * Drawback is that the caller would then have to keep track of
> + * the folio size- and usage.
> + */
> +unsigned long
> +ttm_backup_backup_page(struct ttm_backup *backup, struct page *page,
> +		       bool writeback, pgoff_t idx, gfp_t page_gfp,
> +		       gfp_t alloc_gfp)
> +{
> +	struct file *filp = ttm_backup_to_file(backup);
> +	struct address_space *mapping = filp->f_mapping;
> +	unsigned long handle = 0;
> +	struct folio *to_folio;
> +	int ret;
> +
> +	to_folio = shmem_read_folio_gfp(mapping, idx, alloc_gfp);
> +	if (IS_ERR(to_folio))
> +		return handle;

Just that I sleep better: This can never return a folio larger than a 
page, doesn't it?

Apart from those background questions looks good to me.

Regards,
Christian.

> +
> +	folio_mark_accessed(to_folio);
> +	folio_lock(to_folio);
> +	folio_mark_dirty(to_folio);
> +	copy_highpage(folio_file_page(to_folio, idx), page);
> +	handle = ttm_backup_shmem_idx_to_handle(idx);
> +
> +	if (writeback && !folio_mapped(to_folio) &&
> +	    folio_clear_dirty_for_io(to_folio)) {
> +		struct writeback_control wbc = {
> +			.sync_mode = WB_SYNC_NONE,
> +			.nr_to_write = SWAP_CLUSTER_MAX,
> +			.range_start = 0,
> +			.range_end = LLONG_MAX,
> +			.for_reclaim = 1,
> +		};
> +		folio_set_reclaim(to_folio);
> +		ret = mapping->a_ops->writepage(folio_file_page(to_folio, idx), &wbc);
> +		if (!folio_test_writeback(to_folio))
> +			folio_clear_reclaim(to_folio);
> +		/* If writepage succeeds, it unlocks the folio */
> +		if (ret)
> +			folio_unlock(to_folio);
> +	} else {
> +		folio_unlock(to_folio);
> +	}
> +
> +	folio_put(to_folio);
> +
> +	return handle;
> +}
> +
> +/**
> + * ttm_backup_fini() - Free the struct backup resources after last use.
> + * @backup: Pointer to the struct backup whose resources to free.
> + *
> + * After a call to this function, it's illegal to use the @backup pointer.
> + */
> +void ttm_backup_fini(struct ttm_backup *backup)
> +{
> +	fput(ttm_backup_to_file(backup));
> +}
> +
> +/**
> + * ttm_backup_bytes_avail() - Report the approximate number of bytes of backup space
> + * left for backup.
> + *
> + * This function is intended also for driver use to indicate whether a
> + * backup attempt is meaningful.
> + *
> + * Return: An approximate size of backup space available.
> + */
> +u64 ttm_backup_bytes_avail(void)
> +{
> +	/*
> +	 * The idea behind backing up to shmem is that shmem objects may
> +	 * eventually be swapped out. So no point swapping out if there
> +	 * is no or low swap-space available. But the accuracy of this
> +	 * number also depends on shmem actually swapping out backed-up
> +	 * shmem objects without too much buffering.
> +	 */
> +	return (u64)get_nr_swap_pages() << PAGE_SHIFT;
> +}
> +EXPORT_SYMBOL_GPL(ttm_backup_bytes_avail);
> +
> +/**
> + * ttm_backup_shmem_create() - Create a shmem-based struct backup.
> + * @size: The maximum size (in bytes) to back up.
> + *
> + * Create a backup utilizing shmem objects.
> + *
> + * Return: A pointer to a struct ttm_backup on success,
> + * an error pointer on error.
> + */
> +struct ttm_backup *ttm_backup_shmem_create(loff_t size)
> +{
> +	struct file *filp;
> +
> +	filp = shmem_file_setup("ttm shmem backup", size, 0);
> +
> +	return ttm_file_to_backup(filp);
> +}
> diff --git a/include/drm/ttm/ttm_backup.h b/include/drm/ttm/ttm_backup.h
> new file mode 100644
> index 000000000000..20609da7e281
> --- /dev/null
> +++ b/include/drm/ttm/ttm_backup.h
> @@ -0,0 +1,74 @@
> +/* SPDX-License-Identifier: MIT */
> +/*
> + * Copyright © 2024 Intel Corporation
> + */
> +
> +#ifndef _TTM_BACKUP_H_
> +#define _TTM_BACKUP_H_
> +
> +#include <linux/mm_types.h>
> +#include <linux/shmem_fs.h>
> +
> +struct ttm_backup;
> +
> +/**
> + * ttm_backup_handle_to_page_ptr() - Convert handle to struct page pointer
> + * @handle: The handle to convert.
> + *
> + * Converts an opaque handle received from the
> + * struct ttm_backoup_ops::backup_page() function to an (invalid)
> + * struct page pointer suitable for a struct page array.
> + *
> + * Return: An (invalid) struct page pointer.
> + */
> +static inline struct page *
> +ttm_backup_handle_to_page_ptr(unsigned long handle)
> +{
> +	return (struct page *)(handle << 1 | 1);
> +}
> +
> +/**
> + * ttm_backup_page_ptr_is_handle() - Whether a struct page pointer is a handle
> + * @page: The struct page pointer to check.
> + *
> + * Return: true if the struct page pointer is a handld returned from
> + * ttm_backup_handle_to_page_ptr(). False otherwise.
> + */
> +static inline bool ttm_backup_page_ptr_is_handle(const struct page *page)
> +{
> +	return (unsigned long)page & 1;
> +}
> +
> +/**
> + * ttm_backup_page_ptr_to_handle() - Convert a struct page pointer to a handle
> + * @page: The struct page pointer to convert
> + *
> + * Return: The handle that was previously used in
> + * ttm_backup_handle_to_page_ptr() to obtain a struct page pointer, suitable
> + * for use as argument in the struct ttm_backup_ops drop() or
> + * copy_backed_up_page() functions.
> + */
> +static inline unsigned long
> +ttm_backup_page_ptr_to_handle(const struct page *page)
> +{
> +	WARN_ON(!ttm_backup_page_ptr_is_handle(page));
> +	return (unsigned long)page >> 1;
> +}
> +
> +void ttm_backup_drop(struct ttm_backup *backup, pgoff_t handle);
> +
> +int ttm_backup_copy_page(struct ttm_backup *backup, struct page *dst,
> +			 pgoff_t handle, bool intr);
> +
> +unsigned long
> +ttm_backup_backup_page(struct ttm_backup *backup, struct page *page,
> +		       bool writeback, pgoff_t idx, gfp_t page_gfp,
> +		       gfp_t alloc_gfp);
> +
> +void ttm_backup_fini(struct ttm_backup *backup);
> +
> +u64 ttm_backup_bytes_avail(void);
> +
> +struct ttm_backup *ttm_backup_shmem_create(loff_t size);
> +
> +#endif