From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta0.migadu.com (out-173.mta0.migadu.com [91.218.175.173]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 610C42DA759 for ; Thu, 23 Apr 2026 02:10:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776910241; cv=none; b=QKMGcJjFVMhncDqh+0ebEAczvTti18tRmP5KM+5qXIEPsY06sn39RYXNiaqwxYTa3Tk2/uB4wYZttj88K5brdN1TlcfcYxbZNJrBb/5hxkZ7E3SGz4bWACTn16rGkfR40LYeDaL6xoQr3kXl8cbszydGgQFIZDMj47HiG5fNwec= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776910241; c=relaxed/simple; bh=SSOiNBsQYB//ZyxGnx5tlvfGNd1Ix+ReYfniUyCi3eY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=HG6X44wJVBsXQksTwSO0q3moU+2/vQa1eC6Q9nUcCCWIdTaWzIZkxCC5/S/QY8bgh5HITU0wNmX11L3l7NWw/FMNUydv0GDLY1Gnwe1yUsXtNJE8FaLC8mZYu6TGmV+M5rF8i7RvlDX9kEG/Vt+0XPjJ2UCmhIzgsQVqjQKcRsc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=xPydnDoi; arc=none smtp.client-ip=91.218.175.173 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="xPydnDoi" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1776910235; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=rUP+MiZKx1TpbahvwAghqbXiqiafKtQ76tYAx0gMQDw=; b=xPydnDoi1XVxGml1ouEiNHHuoWDpvGHlgJrIfyBZIPBfz6OyQVifmynH866kSVdgQLUcU4 6awrQxKBtJBSWuQclU3EpB/6zzNAXYGgmB6RMp8LDyRNXW16jh10iFqSVeCzTHKId3HrMg MJSrhKaCoUxgFHL7RYl1Yw7pgA/3QGw= Date: Thu, 23 Apr 2026 10:09:53 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2] mm/alloc_tag: replace fixed-size early PFN array with dynamic linked list To: Suren Baghdasaryan Cc: Kent Overstreet , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260421031406.1189000-1-hao.ge@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Ge In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2026/4/23 01:32, Suren Baghdasaryan wrote: > On Mon, Apr 20, 2026 at 8:15 PM Hao Ge wrote: >> Pages allocated before page_ext is available have their codetag left >> uninitialized. Track these early PFNs and clear their codetag in >> clear_early_alloc_pfn_tag_refs() to avoid "alloc_tag was not set" >> warnings when they are freed later. >> >> Currently a fixed-size array of 8192 entries is used, with a warning if >> the limit is exceeded. However, the number of early allocations depends >> on the number of CPUs and can be larger than 8192. >> >> Replace the fixed-size array with a dynamically allocated linked list. >> Each page is carved into early_pfn_node entries and the remainder is >> kept as a freelist for subsequent allocations. >> >> The list nodes themselves are allocated via alloc_page(), which would >> trigger __pgalloc_tag_add() -> alloc_tag_add_early_pfn() -> >> alloc_early_pfn_node() and recurse indefinitely. Introduce >> __GFP_NO_CODETAG (reuses the %__GFP_NO_OBJ_EXT bit) and pass >> gfp_flags through pgalloc_tag_add() so that the early path can skip >> recording allocations that carry this flag. Hi Suren > Hi Hao, > Thanks for following up on this! Happy to help further develop this feature. Feel free to reach out if there's anything else I can do. > >> Signed-off-by: Hao Ge >> --- >> v2: >> - Use cmpxchg to atomically update early_pfn_pages, preventing page leak under concurrent allocation >> - Pass gfp_flags through the full call chain and use gfpflags_allow_blocking() >> to select GFP_KERNEL vs GFP_ATOMIC, avoiding unnecessary GFP_ATOMIC in process context >> --- >> include/linux/alloc_tag.h | 22 +++++++- >> lib/alloc_tag.c | 102 ++++++++++++++++++++++++++------------ >> mm/page_alloc.c | 29 +++++++---- >> 3 files changed, 108 insertions(+), 45 deletions(-) >> >> diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h >> index 02de2ede560f..2fa695bd3c53 100644 >> --- a/include/linux/alloc_tag.h >> +++ b/include/linux/alloc_tag.h >> @@ -150,6 +150,23 @@ static inline struct alloc_tag_counters alloc_tag_read(struct alloc_tag *tag) >> } >> >> #ifdef CONFIG_MEM_ALLOC_PROFILING_DEBUG >> +/* >> + * Skip early PFN recording for a page allocation. Reuses the >> + * %__GFP_NO_OBJ_EXT bit. Used by alloc_early_pfn_node() to avoid >> + * recursion when allocating pages for the early PFN tracking list >> + * itself. >> + * >> + * Callers must set the codetag to CODETAG_EMPTY (via >> + * clear_page_tag_ref()) before freeing pages allocated with this >> + * flag once page_ext becomes available, otherwise >> + * alloc_tag_sub_check() will trigger a warning. >> + */ >> +#define __GFP_NO_CODETAG __GFP_NO_OBJ_EXT >> + >> +static inline bool should_record_early_pfn(gfp_t gfp_flags) >> +{ >> + return !(gfp_flags & __GFP_NO_CODETAG); >> +} >> static inline void alloc_tag_add_check(union codetag_ref *ref, struct alloc_tag *tag) >> { >> WARN_ONCE(ref && ref->ct && !is_codetag_empty(ref), >> @@ -163,11 +180,12 @@ static inline void alloc_tag_sub_check(union codetag_ref *ref) >> { >> WARN_ONCE(ref && !ref->ct, "alloc_tag was not set\n"); >> } >> -void alloc_tag_add_early_pfn(unsigned long pfn); >> +void alloc_tag_add_early_pfn(unsigned long pfn, gfp_t gfp_flags); >> #else >> static inline void alloc_tag_add_check(union codetag_ref *ref, struct alloc_tag *tag) {} >> static inline void alloc_tag_sub_check(union codetag_ref *ref) {} >> -static inline void alloc_tag_add_early_pfn(unsigned long pfn) {} >> +static inline void alloc_tag_add_early_pfn(unsigned long pfn, gfp_t gfp_flags) {} >> +static inline bool should_record_early_pfn(gfp_t gfp_flags) { return true; } > > If CONFIG_MEM_ALLOC_PROFILING_DEBUG=n why should we record early pfns? Good point! I'll address this in the next patch. >> #endif >> >> /* Caller should verify both ref and tag to be valid */ >> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c >> index ed1bdcf1f8ab..cfc68e397eba 100644 >> --- a/lib/alloc_tag.c >> +++ b/lib/alloc_tag.c >> @@ -766,45 +766,75 @@ static __init bool need_page_alloc_tagging(void) >> * Some pages are allocated before page_ext becomes available, leaving >> * their codetag uninitialized. Track these early PFNs so we can clear >> * their codetag refs later to avoid warnings when they are freed. >> - * >> - * Early allocations include: >> - * - Base allocations independent of CPU count >> - * - Per-CPU allocations (e.g., CPU hotplug callbacks during smp_init, >> - * such as trace ring buffers, scheduler per-cpu data) >> - * >> - * For simplicity, we fix the size to 8192. >> - * If insufficient, a warning will be triggered to alert the user. >> - * >> - * TODO: Replace fixed-size array with dynamic allocation using >> - * a GFP flag similar to ___GFP_NO_OBJ_EXT to avoid recursion. >> */ >> -#define EARLY_ALLOC_PFN_MAX 8192 >> +struct early_pfn_node { >> + struct early_pfn_node *next; >> + unsigned long pfn; >> +}; >> + >> +#define NODES_PER_PAGE (PAGE_SIZE / sizeof(struct early_pfn_node)) >> >> -static unsigned long early_pfns[EARLY_ALLOC_PFN_MAX] __initdata; >> -static atomic_t early_pfn_count __initdata = ATOMIC_INIT(0); >> +static struct early_pfn_node *early_pfn_list __initdata; >> +static struct early_pfn_node *early_pfn_freelist __initdata; >> +static struct page *early_pfn_pages __initdata; > This early_pfn_node linked list seems overly complex. Why not just > allocate a page and use page->lru to place it into a linked list? I > think the code will end up much simpler. Ah, yes! You mentioned this before, but I misunderstood your point. I apologize for the confusion. I'll optimize this in the next revision. > >> -static void __init __alloc_tag_add_early_pfn(unsigned long pfn) >> +static struct early_pfn_node *__init alloc_early_pfn_node(gfp_t gfp_flags) >> { >> - int old_idx, new_idx; >> + struct early_pfn_node *ep, *new; >> + struct page *page, *old_page; >> + gfp_t gfp = gfpflags_allow_blocking(gfp_flags) ? GFP_KERNEL : GFP_ATOMIC; >> + int i; >> + >> +retry: >> + ep = READ_ONCE(early_pfn_freelist); >> + if (ep) { >> + struct early_pfn_node *next = READ_ONCE(ep->next); >> + >> + if (try_cmpxchg(&early_pfn_freelist, &ep, next)) >> + return ep; >> + goto retry; >> + } >> + >> + page = alloc_page(gfp | __GFP_NO_CODETAG | __GFP_ZERO); One more question: since this is called in an RCU context, should we use GFP_ATOMIC by default? Also, should we remove GFP_KSWAPD_RECLAIM here? I'm not entirely sure, but I recall Sashiko mentioned a warning about this before: --- page = alloc_page(GFP_ATOMIC | __GFP_NO_CODETAG | __GFP_ZERO); Sashiko's concerns: Can this lead to a deadlock by introducing lock recursion? alloc_early_pfn_node() is invoked as a post-allocation hook for early boot pages via pgalloc_tag_add(). GFP_ATOMIC includes __GFP_KSWAPD_RECLAIM, which triggers wakeup_kswapd() and acquires scheduler locks. If the original allocation was made under scheduler locks and intentionally stripped __GFP_KSWAPD_RECLAIM to prevent recursion, does this hardcoded GFP_ATOMIC force it back on? Should the hook inherit or constrain its flags based on the caller's gfp_flags instead? --- Even though this only happens during early boot, should we handle it more safely? >> + if (!page) >> + return NULL; >> + >> + new = page_address(page); >> + for (i = 0; i < NODES_PER_PAGE - 1; i++) >> + new[i].next = &new[i + 1]; >> + new[NODES_PER_PAGE - 1].next = NULL; >> + >> + if (cmpxchg(&early_pfn_freelist, NULL, new + 1)) { >> + __free_page(page); >> + goto retry; >> + } >> >> do { >> - old_idx = atomic_read(&early_pfn_count); >> - if (old_idx >= EARLY_ALLOC_PFN_MAX) { >> - pr_warn_once("Early page allocations before page_ext init exceeded EARLY_ALLOC_PFN_MAX (%d)\n", >> - EARLY_ALLOC_PFN_MAX); >> - return; >> - } >> - new_idx = old_idx + 1; >> - } while (!atomic_try_cmpxchg(&early_pfn_count, &old_idx, new_idx)); >> + old_page = READ_ONCE(early_pfn_pages); >> + page->private = (unsigned long)old_page; >> + } while (cmpxchg(&early_pfn_pages, old_page, page) != old_page); > I don't think this whole lockless schema is worth the complexity. > alloc_early_pfn_node() is called only during early init and is called > perhaps a few hundred times in total. Why not use a simple spinlock to > synchronize this operation and be done with it? I initially used a simple spinlock, but Sashiko raised a good point in his review: https://sashiko.dev/#/patchset/20260319083153.2488005-1-hao.ge%40linux.dev Since alloc_early_pfn_node() is called during early init from an unknown context, in the early page allocation path, a lockless approach is safer. However, you're right that if we use page->lru as a linked list (which you suggested earlier), the code becomes much simpler. I plan to simplify the code and continue using the lockless approach in the next version. >> + >> + return new; >> +} >> + >> +static void __init __alloc_tag_add_early_pfn(unsigned long pfn, gfp_t gfp_flags) >> +{ >> + struct early_pfn_node *ep = alloc_early_pfn_node(gfp_flags); >> >> - early_pfns[old_idx] = pfn; >> + if (!ep) >> + return; >> + >> + ep->pfn = pfn; >> + do { >> + ep->next = READ_ONCE(early_pfn_list); >> + } while (!try_cmpxchg(&early_pfn_list, &ep->next, ep)); >> } >> >> -typedef void alloc_tag_add_func(unsigned long pfn); >> +typedef void alloc_tag_add_func(unsigned long pfn, gfp_t gfp_flags); >> static alloc_tag_add_func __rcu *alloc_tag_add_early_pfn_ptr __refdata = >> RCU_INITIALIZER(__alloc_tag_add_early_pfn); >> >> -void alloc_tag_add_early_pfn(unsigned long pfn) >> +void alloc_tag_add_early_pfn(unsigned long pfn, gfp_t gfp_flags) >> { >> alloc_tag_add_func *alloc_tag_add; >> >> @@ -814,13 +844,14 @@ void alloc_tag_add_early_pfn(unsigned long pfn) >> rcu_read_lock(); >> alloc_tag_add = rcu_dereference(alloc_tag_add_early_pfn_ptr); >> if (alloc_tag_add) >> - alloc_tag_add(pfn); >> + alloc_tag_add(pfn, gfp_flags); >> rcu_read_unlock(); >> } >> >> static void __init clear_early_alloc_pfn_tag_refs(void) >> { >> - unsigned int i; >> + struct early_pfn_node *ep; >> + struct page *page, *next; >> >> if (static_key_enabled(&mem_profiling_compressed)) >> return; >> @@ -829,14 +860,13 @@ static void __init clear_early_alloc_pfn_tag_refs(void) >> /* Make sure we are not racing with __alloc_tag_add_early_pfn() */ >> synchronize_rcu(); >> >> - for (i = 0; i < atomic_read(&early_pfn_count); i++) { >> - unsigned long pfn = early_pfns[i]; >> + for (ep = early_pfn_list; ep; ep = ep->next) { >> >> - if (pfn_valid(pfn)) { >> - struct page *page = pfn_to_page(pfn); >> + if (pfn_valid(ep->pfn)) { >> union pgtag_ref_handle handle; >> union codetag_ref ref; >> >> + page = pfn_to_page(ep->pfn); >> if (get_page_tag_ref(page, &ref, &handle)) { >> /* >> * An early-allocated page could be freed and reallocated >> @@ -861,6 +891,12 @@ static void __init clear_early_alloc_pfn_tag_refs(void) >> } >> >> } >> + >> + for (page = early_pfn_pages; page; page = next) { >> + next = (struct page *)page->private; >> + clear_page_tag_ref(page); >> + __free_page(page); >> + } >> } >> #else /* !CONFIG_MEM_ALLOC_PROFILING_DEBUG */ >> static inline void __init clear_early_alloc_pfn_tag_refs(void) {} >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 04494bc2e46f..4e2bfb3714e1 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1284,7 +1284,7 @@ void __clear_page_tag_ref(struct page *page) >> /* Should be called only if mem_alloc_profiling_enabled() */ >> static noinline >> void __pgalloc_tag_add(struct page *page, struct task_struct *task, >> - unsigned int nr) >> + unsigned int nr, gfp_t gfp_flags) >> { >> union pgtag_ref_handle handle; >> union codetag_ref ref; >> @@ -1294,21 +1294,30 @@ void __pgalloc_tag_add(struct page *page, struct task_struct *task, >> update_page_tag_ref(handle, &ref); >> put_page_tag_ref(handle); >> } else { >> - /* >> - * page_ext is not available yet, record the pfn so we can >> - * clear the tag ref later when page_ext is initialized. >> - */ >> - alloc_tag_add_early_pfn(page_to_pfn(page)); >> + >> if (task->alloc_tag) >> alloc_tag_set_inaccurate(task->alloc_tag); >> + >> + /* >> + * page_ext is not available yet, skip if this allocation >> + * doesn't need early PFN recording. >> + */ >> + if (unlikely(!should_record_early_pfn(gfp_flags))) >> + return; >> + >> + /* >> + * Record the pfn so the tag ref can be cleared later >> + * when page_ext is initialized. >> + */ >> + alloc_tag_add_early_pfn(page_to_pfn(page), gfp_flags); > nit: This seems shorter and more readable: > > if (unlikely(should_record_early_pfn(gfp_flags))) > alloc_tag_add_early_pfn(page_to_pfn(page), > gfp_flags); OK, will improve it in the next version. Thanks for taking the time to review and provide these suggestions! Thanks Best Regards Hao >> } >> } >> >> static inline void pgalloc_tag_add(struct page *page, struct task_struct *task, >> - unsigned int nr) >> + unsigned int nr, gfp_t gfp_flags) >> { >> if (mem_alloc_profiling_enabled()) >> - __pgalloc_tag_add(page, task, nr); >> + __pgalloc_tag_add(page, task, nr, gfp_flags); >> } >> >> /* Should be called only if mem_alloc_profiling_enabled() */ >> @@ -1341,7 +1350,7 @@ static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr) >> #else /* CONFIG_MEM_ALLOC_PROFILING */ >> >> static inline void pgalloc_tag_add(struct page *page, struct task_struct *task, >> - unsigned int nr) {} >> + unsigned int nr, gfp_t gfp_flags) {} >> static inline void pgalloc_tag_sub(struct page *page, unsigned int nr) {} >> static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr) {} >> >> @@ -1896,7 +1905,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order, >> >> set_page_owner(page, order, gfp_flags); >> page_table_check_alloc(page, order); >> - pgalloc_tag_add(page, current, 1 << order); >> + pgalloc_tag_add(page, current, 1 << order, gfp_flags); >> } >> >> static void prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags, >> -- >> 2.25.1 >>