Re: [PATCH v4] mm: shmem: implement POSIX_FADV_[WILL|DONT]NEED for shmem

[Charan Teja Kalla]

Just a gentle ping. Please help in providing your inputs here.

Thanks,

On 2/11/2022 3:04 PM, Charan Teja Kalla wrote:
> Currently fadvise(2) is supported only for the files that doesn't
> associated with noop_backing_dev_info thus for the files, like shmem,
> fadvise results into NOP. But then there is file_operations->fadvise()
> that lets the file systems to implement their own fadvise
> implementation. Use this support to implement some of the POSIX_FADV_XXX
> functionality for shmem files.
> 
> This patch aims to implement POSIX_FADV_WILLNEED and POSIX_FADV_DONTNEED
> advices to shmem files which can be helpful for the drivers who may want
> to manage the shmem pages of the files that are created through
> shmem_file_setup[_with_mnt]().  An example usecase may be like, driver
> can create the shmem file of the size equal to its requirements and
> map the pages for DMA and then pass the fd to user. The user who knows
> well about the usage of these pages can now decide when these pages are
> not required push them to swap through DONTNEED thus free up memory well
> in advance rather than relying on the reclaim and use WILLNEED when it
> decide that they are useful in the near future. IOW, it lets the clients
> to free up/read the memory when it wants to. Another usecase is that GEM
> objets which are currenlty allocated and managed through shmem files can
> use vfs_fadvise(DONT|WILLNEED) on shmem fd when the driver comes to
> know(like through some hints from user space) that GEM objects are not
> going to use/will need in the near future.
> 
> Some questions asked while reviewing this patch:
> 
> Q) Can the same thing be achieved with FD mapped to user and use
> madvise?
> A) All drivers are not mapping all the shmem fd's to user space and want
> to manage them with in the kernel. Ex: shmem memory can be mapped to the
> other subsystems and they fill in the data and then give it to other
> subsystem for further processing, where, the user mapping is not at all
> required.  A simple example, memory that is given for gpu subsystem
> which can be filled directly and give to display subsystem. And the
> respective drivers know well about when to keep that memory in ram or
> swap based on may be a user activity.
> 
> Q) Should we add the documentation section in Manual pages?
> A) The man[1] pages for the fadvise() whatever says is also applicable
> for shmem files. so couldn't feel it correct to add specific to shmem
> files separately.
> [1] https://linux.die.net/man/2/fadvise
> 
> Q) The proposed semantics of POSIX_FADV_DONTNEED is actually similar to
> MADV_PAGEOUT and different from MADV_DONTNEED. This is a user facing API
> and this difference will cause confusion?
> A) man pages [1] says that "POSIX_FADV_DONTNEED attempts to free cached
> pages associated with the specified region." This means on issuing this
> FADV, it is expected to free the file cache pages. And it is
> implementation defined If the dirty pages may be attempted to writeback.
> And the unwritten dirty pages will not be freed. So, FADV_DONTNEED also
> covers the semantics of MADV_PAGEOUT for file pages and there is no
> purpose of PAGEOUT for file pages.
> [1] https://man7.org/linux/man-pages/man2/posix_fadvise.2.html
> 
> Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
> ---
> Changes in V4:
>   -- Changed the code to use reclaim_pages() to writeout the shmem pages to swap and then reclaim.
>   -- Addressed comments from Mark Hemment and Matthew.
>   -- fadvise() on shmem file may even unmap a page.
> 
> Changes in V3:
>   -- Considered THP pages while doing FADVISE_[DONT|WILL]NEED, identified by Matthew.
>   -- xarray used properly, as identified by Matthew.
>   -- Excluded mapped pages as it requires unmapping and the man pages of fadvise don't talk about them.
>   -- RESEND: Fixed the compilation issue when CONFIG_TMPFS is not defined.
>   -- https://patchwork.kernel.org/project/linux-mm/patch/1641488717-13865-1-git-send-email-quic_charante@xxxxxxxxxxx/
> 
> Changes in V2:
>   -- Rearranged the code to not to sleep with rcu_lock while using xas_() functionality.
>   -- Addressed the comments from Suren.
>   -- https://patchwork.kernel.org/project/linux-mm/patch/1638442253-1591-1-git-send-email-quic_charante@xxxxxxxxxxx/
> 
> changes in V1:
>   -- Created the interface for fadvise(2) to work on shmem files.
>   -- https://patchwork.kernel.org/project/linux-mm/patch/1633701982-22302-1-git-send-email-charante@xxxxxxxxxxxxxx/
> 
>  mm/shmem.c | 129 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 129 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 18f93c2..fe475af 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -39,6 +39,9 @@
>  #include <linux/frontswap.h>
>  #include <linux/fs_parser.h>
>  #include <linux/swapfile.h>
> +#include <linux/mm_inline.h>
> +#include <linux/fadvise.h>
> +#include <linux/page_idle.h>
>  
>  static struct vfsmount *shm_mnt;
>  
> @@ -2275,6 +2278,131 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
>  	return 0;
>  }
>  
> +static void shmem_isolate_pages_range(struct address_space *mapping, loff_t start,
> +				loff_t end, struct list_head *list)
> +{
> +	XA_STATE(xas, &mapping->i_pages, start);
> +	struct page *page;
> +
> +	rcu_read_lock();
> +	xas_for_each(&xas, page, end) {
> +		if (xas_retry(&xas, page))
> +			continue;
> +		if (xa_is_value(page))
> +			continue;
> +
> +		if (!get_page_unless_zero(page))
> +			continue;
> +		if (isolate_lru_page(page)) {
> +			put_page(page);
> +			continue;
> +		}
> +		put_page(page);
> +
> +		if (PageUnevictable(page) || page_mapcount(page) > 1) {
> +			putback_lru_page(page);
> +			continue;
> +		}
> +
> +		/*
> +		 * Prepare the page to be passed to the reclaim_pages().
> +		 * VM couldn't reclaim the page unless we clear PG_young.
> +		 * Also, to ensure that the pages are written before
> +		 * reclaiming, page is set to dirty.
> +		 * Since we are not clearing the pte_young in the mapped
> +		 * page pte's, its reclaim may not be attempted.
> +		 */
> +		ClearPageReferenced(page);
> +		test_and_clear_page_young(page);
> +		SetPageDirty(page);
> +		list_add(&page->lru, list);
> +		if (need_resched()) {
> +			xas_pause(&xas);
> +			cond_resched_rcu();
> +		}
> +	}
> +	rcu_read_unlock();
> +}
> +
> +static int shmem_fadvise_dontneed(struct address_space *mapping, loff_t start,
> +				loff_t end)
> +{
> +	LIST_HEAD(list);
> +
> +	if (!shmem_mapping(mapping))
> +		return -EINVAL;
> +
> +	if (!total_swap_pages)
> +		return 0;
> +
> +	lru_add_drain();
> +	shmem_isolate_pages_range(mapping, start, end, &list);
> +	reclaim_pages(&list);
> +
> +	return 0;
> +}
> +
> +static int shmem_fadvise_willneed(struct address_space *mapping,
> +				 pgoff_t start, pgoff_t long end)
> +{
> +	struct page *page;
> +	pgoff_t index;
> +
> +	xa_for_each_range(&mapping->i_pages, index, page, start, end) {
> +		if (!xa_is_value(page))
> +			continue;
> +		page = shmem_read_mapping_page(mapping, index);
> +		if (!IS_ERR(page))
> +			put_page(page);
> +	}
> +
> +	return 0;
> +}
> +
> +static int shmem_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
> +{
> +	loff_t endbyte;
> +	pgoff_t start_index;
> +	pgoff_t end_index;
> +	struct address_space *mapping;
> +	int ret = 0;
> +
> +	mapping = file->f_mapping;
> +	if (!mapping || len < 0)
> +		return -EINVAL;
> +
> +	endbyte = (u64)offset + (u64)len;
> +	if (!len || endbyte < len)
> +		endbyte = -1;
> +	else
> +		endbyte--;
> +
> +
> +	start_index = offset >> PAGE_SHIFT;
> +	end_index   = endbyte >> PAGE_SHIFT;
> +	switch (advice) {
> +	case POSIX_FADV_DONTNEED:
> +		ret = shmem_fadvise_dontneed(mapping, start_index, end_index);
> +		break;
> +	case POSIX_FADV_WILLNEED:
> +		ret = shmem_fadvise_willneed(mapping, start_index, end_index);
> +		break;
> +	case POSIX_FADV_NORMAL:
> +	case POSIX_FADV_RANDOM:
> +	case POSIX_FADV_SEQUENTIAL:
> +	case POSIX_FADV_NOREUSE:
> +		/*
> +		 * No bad return value, but ignore advice. May have to
> +		 * implement in future.
> +		 */
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return ret;
> +}
> +
>  static struct inode *shmem_get_inode(struct super_block *sb, const struct inode *dir,
>  				     umode_t mode, dev_t dev, unsigned long flags)
>  {
> @@ -3777,6 +3905,7 @@ static const struct file_operations shmem_file_operations = {
>  	.splice_write	= iter_file_splice_write,
>  	.fallocate	= shmem_fallocate,
>  #endif
> +	.fadvise	= shmem_fadvise,
>  };
>  
>  static const struct inode_operations shmem_inode_operations = {