Thanks Minchan for the review!! On 4/7/2023 5:14 AM, Minchan Kim wrote: > On Tue, Feb 14, 2023 at 06:21:50PM +0530, 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 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 >> 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> > > I am not familar with why the shmem has noop_backing_dev_info > but the below code to reclaim shmem pages and POXIS_FADV_DONTNEED > semantic looks correct for me. > Thanks!! > Only nit is the description covers mostly DONTNEED case but not > WILLNEED case.Okay. How about adding the below to the end of the 2nd paragraph of the commit message: Application that does require the reclaimed pages, say when the app put to foreground, can issue the POSIX_FADV_WILLNEED to bring back them from the swap area. Alternatively the drivers can also use shmem_read_mapping_page_gfp() to bring back the reclaimed shmem pages. @Andrew: I am not sure If this update to commit message requires respin of the patchset. Please let me know If it required so. Thanks, Charan
