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From: Manfred Spraul <manfred@colorfullife.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	dave@stgolabs.net, mm-commits@vger.kernel.org,
	willy@infradead.org
Cc: 1vier1@web.de
Subject: Re: + ipc-convert-ipcs_idr-to-xarray-update.patch added to -mm tree
Date: Wed, 30 Dec 2020 16:44:30 +0100
Message-ID: <3e33fff1-4b28-8fe5-6567-3f26cdc8b1b2@colorfullife.com> (raw)
In-Reply-To: <20200424014753.DfBuzjmzo%akpm@linux-foundation.org>

Hi Matthew,

Due to lack of time, I didn't follow up immediately, but I haven't 
forgotten the patch series:

On 4/24/20 3:47 AM, Andrew Morton wrote:
> The patch titled
>       Subject: ipc-convert-ipcs_idr-to-xarray-update

[...]

> -		xas_lock(&xas);
> +		xa_lock(&ids->ipcs);
>   
> -		min_idx = ids->next_idx;
> -		new->seq = ids->seq;
> +		err = __xa_alloc_cyclic(&ids->ipcs, &idx, NULL,
> +				XA_LIMIT(0, max_idx), &ids->next_idx,
> +				GFP_KERNEL);
> +		if (err == 1) {
> +			ids->seq++;
> +			if (ids->seq >= ipcid_seq_max())
> +				ids->seq = 0;
> +		}
> +
> +		if (err >= 0) {
> +			new->seq = ids->seq;
> +			new->id = (new->seq << ipcmni_seq_shift()) + idx;
> +			/* xa_store contains a write barrier */
> +			__xa_store(&ids->ipcs, idx, new, GFP_KERNEL);
> +		}

The GFP_KERNEL is really ugly.

When I see "GFP_KERNEL", it means for me that the called function might 
sleep.
When I see "GFP_ATOMIC", it means for me that the called function may 
fail, and that the caller must implement a retry algorithm.

I think the GFP_KERNEL here means nothing, but I'm not sure.

Would it be possible to make __xa_store(,,,0) an officially supported 
feature?
- The function will fail if there was no previous xa_alloc for exactly 
the to be stored entry (or an xa_insert, or an xa_store(,,,GFP_KERNEL), 
etc.)
- The function will not allocate any memory.
- The function will not fail due to out of memory.
- The function will not drop the xa_lock()

Then the usage would be clear:
    xa_alloc(,GFP_KERNEL)
    acquire_all_locks()
    __xa_store(,,,0)
    drop_all_locks()


The kerneldoc is for me not clear enough:

> **
>  * __xa_store() - Store this entry in the XArray.
>  * @xa: XArray.
>  * @index: Index into array.
>  * @entry: New entry.
>  * @gfp: Memory allocation flags.
>  *
>  * You must already be holding the xa_lock when calling this function.
>  * It will drop the lock if needed to allocate memory, and then reacquire
>  * it afterwards.
Is it guaranteed the dropping/acquiring the lock will never happen after 
storing the entry into the array?
If the xarray code first stores the entry, and then allocates memory for 
tree balancing or similar, then it would break the ipc/*.c

--

     Manfred


  parent reply index

Thread overview: 71+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-21  1:13 incoming Andrew Morton
2020-04-21  1:13 ` [patch 01/15] sh: fix build error in mm/init.c Andrew Morton
2020-04-21  1:13 ` [patch 02/15] slub: avoid redzone when choosing freepointer location Andrew Morton
2020-04-21  1:13 ` [patch 03/15] mm/userfaultfd: disable userfaultfd-wp on x86_32 Andrew Morton
2020-04-21  1:13 ` [patch 04/15] MAINTAINERS: add an entry for kfifo Andrew Morton
2020-04-21  1:13 ` [patch 05/15] mm/hugetlb: fix a addressing exception caused by huge_pte_offset Andrew Morton
2020-04-21  1:13 ` [patch 06/15] mm, gup: return EINTR when gup is interrupted by fatal signals Andrew Morton
2020-04-21  1:13 ` [patch 07/15] checkpatch: fix a typo in the regex for $allocFunctions Andrew Morton
2020-04-21  1:14 ` [patch 08/15] tools/build: tweak unused value workaround Andrew Morton
2020-04-21  1:14 ` [patch 09/15] mm/ksm: fix NULL pointer dereference when KSM zero page is enabled Andrew Morton
2020-04-21  1:14 ` [patch 10/15] mm/shmem: fix build without THP Andrew Morton
2020-04-21  1:14 ` [patch 11/15] vmalloc: fix remap_vmalloc_range() bounds checks Andrew Morton
2020-04-21  1:14 ` [patch 12/15] shmem: fix possible deadlocks on shmlock_user_lock Andrew Morton
2020-04-21  1:14 ` [patch 13/15] mm: shmem: disable interrupt when acquiring info->lock in userfaultfd_copy path Andrew Morton
2020-04-21  1:14 ` [patch 14/15] coredump: fix null pointer dereference on coredump Andrew Morton
2020-04-21  1:14 ` [patch 15/15] tools/vm: fix cross-compile build Andrew Morton
2020-04-21  2:00 ` + mm-memory_hotplug-refrain-from-adding-memory-into-an-impossible-node.patch added to -mm tree Andrew Morton
2020-04-21  2:48 ` + x86-mm-define-mm_p4d_folded.patch " Andrew Morton
2020-04-21  2:52 ` + mm-debug-add-tests-validating-architecture-page-table-helpers-v17.patch " Andrew Morton
2020-04-21  2:59 ` + mm-mmapc-add-more-sanity-checks-to-get_unmapped_area.patch " Andrew Morton
2020-04-21  2:59 ` + mm-mmapc-do-not-allow-mappings-outside-of-allowed-limits.patch " Andrew Morton
2020-04-21  3:07 ` + initrdmem=-option-to-specify-initrd-physical-address-checkpatch-fixes.patch " Andrew Morton
2020-04-21  3:58 ` + initrdmem=-option-to-specify-initrd-physical-address.patch " Andrew Morton
2020-04-21  5:43 ` mmotm 2020-04-20-22-43 uploaded Andrew Morton
2020-04-22  1:36 ` + mm-swapfilec-found_free-could-be-represented-by-tmp-max.patch added to -mm tree Andrew Morton
2020-04-22  1:36 ` + mm-swapfilec-tmp-is-always-smaller-than-max.patch " Andrew Morton
2020-04-22  1:36 ` + mm-swapfilec-omit-a-duplicate-code-by-compare-tmp-and-max-first.patch " Andrew Morton
2020-04-23 22:36 ` + kasan-initialise-array-in-kasan_memcmp-test.patch " Andrew Morton
2020-04-23 22:38 ` + kvm-svm-change-flag-passed-to-gup-fast-in-sev_pin_memory.patch " Andrew Morton
2020-04-23 22:41 ` + mm-pass-task-and-mm-to-do_madvise-fix.patch " Andrew Morton
2020-04-23 22:44 ` + mm-support-vector-address-ranges-for-process_madvise.patch " Andrew Morton
2020-04-23 22:44 ` + mm-support-vector-address-ranges-for-process_madvise-fix.patch " Andrew Morton
2020-04-23 22:48 ` + kasan-stop-tests-being-eliminated-as-dead-code-with-fortify_source.patch " Andrew Morton
2020-04-23 22:48 ` + stringh-fix-incompatibility-between-fortify_source-and-kasan.patch " Andrew Morton
2020-04-23 23:03 ` + powerpc-add-support-for-folded-p4d-page-tables-fix.patch " Andrew Morton
2020-04-23 23:09 ` [folded-merged] memcg-optimize-memorynuma_stat-like-memorystat-fix.patch removed from " Andrew Morton
2020-04-23 23:32 ` + slub-remove-userspace-notifier-for-cache-add-remove.patch added to " Andrew Morton
2020-04-23 23:35 ` + ocfs2-mount-shared-volume-without-ha-stack.patch " Andrew Morton
2020-04-24  0:29 ` + mm-memory_hotplug-handle-memblocks-only-with-config_arch_keep_memblock.patch " Andrew Morton
2020-04-24  1:17 ` + mm-return-true-in-cpupid_pid_unset.patch " Andrew Morton
2020-04-24  1:20 ` + kernel-better-document-the-use_mm-unuse_mm-api-contract-v2-fix.patch " Andrew Morton
2020-04-24  1:40 ` + mm-thp-rename-pmd_mknotpresent-as-pmd_mkinvalid-v2.patch " Andrew Morton
2020-04-24  1:47 ` + ipc-convert-ipcs_idr-to-xarray-update.patch " Andrew Morton
     [not found]   ` <20200605195848.GB5393@lca.pw>
     [not found]     ` <20200605201134.GJ19604@bombadil.infradead.org>
2020-06-05 21:20       ` Andrew Morton
2020-12-30 15:44   ` Manfred Spraul [this message]
2020-04-24  2:06 ` + powerpc-spufs-simplify-spufs-core-dumping.patch " Andrew Morton
2020-04-24  2:06 ` + signal-factor-copy_siginfo_to_external32-from-copy_siginfo_to_user32.patch " Andrew Morton
2020-04-24  2:06 ` + binfmt_elf-femove-the-set_fs-in-fill_siginfo_note.patch " Andrew Morton
2020-04-24  2:06 ` + binfmt_elf-remove-the-set_fskernel_ds-in-elf_core_dump.patch " Andrew Morton
2020-04-24  2:06 ` + binfmt_elf_fdpic-remove-the-set_fskernel_ds-in-elf_fdpic_core_dump.patch " Andrew Morton
2020-04-24  2:06 ` + exec-simplify-the-copy_strings_kernel-calling-convention.patch " Andrew Morton
2020-04-24  2:06 ` + exec-open-code-copy_string_kernel.patch " Andrew Morton
2020-04-24  3:24 ` + add-kernel-config-option-for-twisting-kernel-behavior.patch " Andrew Morton
2020-04-24  3:24 ` + twist-allow-disabling-k_spec-function-in-drivers-tty-vt-keyboardc.patch " Andrew Morton
2020-04-24  3:24 ` + twist-add-option-for-selecting-twist-options-for-syzkallers-testing.patch " Andrew Morton
2020-04-24  3:32 ` + eventpoll-fix-missing-wakeup-for-ovflist-in-ep_poll_callback.patch " Andrew Morton
2020-04-24  3:49 ` [obsolete] linux-next-rejects.patch removed from " Andrew Morton
2020-04-24  3:51 ` + mips-mm-add-page-soft-dirty-tracking.patch added to " Andrew Morton
2020-04-24 23:36 ` + mm-memory_hotplug-set-node_start_pfn-of-hotadded-pgdat-to-0.patch " Andrew Morton
2020-04-26  0:09 ` + mm-switch-the-test_vmalloc-module-to-use-__vmalloc_node-fix-fix.patch " Andrew Morton
2020-04-26  0:17 ` + mm-hugetlb-avoid-unnecessary-check-on-pud-and-pmd-entry-in-huge_pte_offset.patch " Andrew Morton
2020-04-26  0:29 ` + eventpoll-fix-missing-wakeup-for-ovflist-in-ep_poll_callback-v2.patch " Andrew Morton
2020-04-26  0:41 ` [withdrawn] kasan-initialise-array-in-kasan_memcmp-test.patch removed from " Andrew Morton
2020-04-26  0:41 ` + kasan-stop-tests-being-eliminated-as-dead-code-with-fortify_source-v4.patch added to " Andrew Morton
2020-04-26  0:48 ` + checkpatch-test-git_dir-changes.patch " Andrew Morton
2020-04-26  1:06 ` + mm-add-debug_wx-support.patch " Andrew Morton
2020-04-26  1:06 ` + riscv-support-debug_wx.patch " Andrew Morton
2020-04-26  1:06 ` + riscv-support-debug_wx-fix.patch " Andrew Morton
2020-04-26  1:06 ` + x86-mm-use-arch_has_debug_wx-instead-of-arch-defined.patch " Andrew Morton
2020-04-26  1:06 ` + arm64-mm-use-arch_has_debug_wx-instead-of-arch-defined.patch " Andrew Morton
2020-04-26  1:09 ` [folded-merged] initrdmem=-option-to-specify-initrd-physical-address-checkpatch-fixes.patch removed from " Andrew Morton

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