Hi Charan, On Mon, Feb 6, 2023 at 6:22 AM Charan Teja Kalla <quic_charante@xxxxxxxxxxx> 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 clients who may want > to manage the shmem pages of the files that are created through > shmem_file_setup[_with_mnt](). One usecase is implemented on the > Snapdragon SoC's running Android where the graphics client is allocating > lot of shmem pages per process and pinning them. When this process is > put to background, the instantaneous reclaim is performed on those shmem > pages using the logic implemented downstream[3][4]. With this patch, the Thanks for upstreaming this feature! > client can now issue the fadvise calls on the shmem files that does the > instantaneous reclaim which can aid the use cases like mentioned above. > > This usecase lead to ~2% reduction in average launch latencies of the > apps and 10% in total number of kills by the low memory killer running > on Android. > > 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. > > 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 [2] 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://linux.die.net/man/2/fadvise > [2] https://man7.org/linux/man-pages/man2/posix_fadvise.2.html > [3] https://git.codelinaro.org/clo/la/platform/vendor/qcom/opensource/graphics-kernel/-/blob/gfx-kernel.lnx.1.0.r3-rel/kgsl_reclaim.c#L289 > [4] https://android.googlesource.com/kernel/common/+/refs/heads/android12-5.10/mm/shmem.c#4310 > > Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx> > --- > mm/shmem.c | 115 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 115 insertions(+) > > diff --git a/mm/shmem.c b/mm/shmem.c > index 0005ab2..58aa3d7 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -39,6 +39,9 @@ > #include <linux/fs_parser.h> > #include <linux/swapfile.h> > #include <linux/iversion.h> > +#include <linux/mm_inline.h> > +#include <linux/fadvise.h> > +#include <linux/page_idle.h> > #include "swap.h" > > static struct vfsmount *shm_mnt; > @@ -2327,6 +2330,117 @@ static void shmem_set_inode_flags(struct inode *inode, unsigned int fsflags) > #define shmem_initxattrs NULL > #endif > > +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 folio *folio; > + > + rcu_read_lock(); > + xas_for_each(&xas, folio, end) { > + if (xas_retry(&xas, folio)) > + continue; > + if (xa_is_value(folio)) > + continue; > + > + if (!folio_try_get(folio)) > + continue; > + if (folio_test_unevictable(folio) || folio_mapped(folio) || > + folio_isolate_lru(folio)) { > + folio_put(folio); > + continue; > + } > + folio_put(folio); > + > + /* > + * Prepare the page to be passed to the reclaim_pages(). > + * VM couldn't reclaim the page unless we clear PG_young. nit: Since you operate on folios now, should you update this comment as well? > + */ > + folio_clear_referenced(folio); > + folio_test_clear_young(folio); > + list_add(&folio->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(folio_list); > + > + if (!total_swap_pages) > + return 0; > + > + lru_add_drain(); > + shmem_isolate_pages_range(mapping, start, end, &folio_list); > + reclaim_pages(&folio_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; > + struct inode *inode = file_inode(file); > + int ret = 0; > + > + if (S_ISFIFO(inode->i_mode)) > + return -ESPIPE; > + > + mapping = file->f_mapping; > + if (!mapping || len < 0 || !shmem_mapping(mapping)) > + return -EINVAL; > + > + endbyte = fadvise_calc_endbyte(offset, len); > + > + start_index = offset >> PAGE_SHIFT; > + end_index = endbyte >> PAGE_SHIFT; > + switch (advice) { > + case POSIX_FADV_DONTNEED: Should (SHMEM_I(inode)->flags & VM_LOCKED) be checked here too? > + 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. > + */ > + break; > + default: > + return -EINVAL; > + } > + > + return ret; > +} > + > static struct inode *shmem_get_inode(struct super_block *sb, struct inode *dir, > umode_t mode, dev_t dev, unsigned long flags) > { > @@ -3933,6 +4047,7 @@ 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 = { > -- > 2.7.4 >
