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Date: Mon, 25 Oct 2021 19:47:06 +0530
From: Charan Teja Kalla <charante@...eaurora.org>
To: hughd@...gle.com, akpm@...ux-foundation.org, vbabka@...e.cz,
rientjes@...gle.com, david@...hat.com, mhocko@...e.com,
linux-mm@...ck.org, ",surenb"@google.com
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: shmem: implement POSIX_FADV_[WILL|DONT]NEED for shmem
Hello,
Can you please provide your valuable feed back here. There are of great
help to me.
Thanks,
Charan
On 10/8/2021 7:36 PM, Charan Teja Reddy 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.
>
> Signed-off-by: Charan Teja Reddy <charante@...eaurora.org>
> ---
> mm/shmem.c | 139 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 139 insertions(+)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 70d9ce2..ab7ea33 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -38,6 +38,8 @@
> #include <linux/hugetlb.h>
> #include <linux/frontswap.h>
> #include <linux/fs_parser.h>
> +#include <linux/mm_inline.h>
> +#include <linux/fadvise.h>
>
> #include <asm/tlbflush.h> /* for arch/microblaze update_mmu_cache() */
>
> @@ -2792,6 +2794,142 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
> return error;
> }
>
> +static int shmem_fadvise_dontneed(struct address_space *mapping, loff_t start,
> + loff_t end)
> +{
> + int ret;
> + struct writeback_control wbc = {
> + .sync_mode = WB_SYNC_NONE,
> + .nr_to_write = LONG_MAX,
> + .range_start = 0,
> + .range_end = LLONG_MAX,
> + .for_reclaim = 1,
> + };
> + struct page *page;
> +
> + XA_STATE(xas, &mapping->i_pages, start);
> + if (!shmem_mapping(mapping))
> + return -EINVAL;
> +
> + if (!total_swap_pages)
> + return 0;
> +
> + lru_add_drain();
> +
> + rcu_read_lock();
> + xas_for_each(&xas, page, end) {
> + if (xas_retry(&xas, page))
> + continue;
> + if (xa_is_value(page))
> + continue;
> + if (isolate_lru_page(page))
> + continue;
> +
> + inc_node_page_state(page, NR_ISOLATED_ANON +
> + page_is_file_lru(page));
> + lock_page(page);
> + ClearPageDirty(page);
> + SetPageReclaim(page);
> + ret = shmem_writepage(page, &wbc);
> + if (!PageWriteback(page))
> + ClearPageReclaim(page);
> + if (ret) {
> + unlock_page(page);
> + putback_lru_page(page);
> + dec_node_page_state(page, NR_ISOLATED_ANON +
> + page_is_file_lru(page));
> + continue;
> + }
> +
> + /*
> + * shmem_writepage() place the page in the swapcache.
> + * Delete the page from the swapcache and release the
> + * page.
> + */
> + lock_page(page);
> + delete_from_swap_cache(page);
> + unlock_page(page);
> + dec_node_page_state(page, NR_ISOLATED_ANON +
> + page_is_file_lru(page));
> + put_page(page);
> + if (need_resched()) {
> + xas_pause(&xas);
> + cond_resched_rcu();
> + }
> + }
> + rcu_read_unlock();
> +
> + return 0;
> +}
> +
> +static int shmem_fadvise_willneed(struct address_space *mapping,
> + pgoff_t start, pgoff_t long end)
> +{
> + struct page *page;
> +
> + XA_STATE(xas, &mapping->i_pages, start);
> + rcu_read_lock();
> + xas_for_each(&xas, page, end) {
> + if (!xa_is_value(page))
> + continue;
> + page = shmem_read_mapping_page(mapping, xas.xa_index);
> + if (!IS_ERR(page))
> + put_page(page);
> +
> + if (need_resched()) {
> + xas_pause(&xas);
> + cond_resched_rcu();
> + }
> + }
> + rcu_read_unlock();
> +
> + 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 int shmem_statfs(struct dentry *dentry, struct kstatfs *buf)
> {
> struct shmem_sb_info *sbinfo = SHMEM_SB(dentry->d_sb);
> @@ -3799,6 +3937,7 @@ 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 = {
>
--
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