On Wed, Dec 20, 2023 at 8:53 PM Fangrui Song <maskray@xxxxxxxxxx> wrote: > > On Wed, Dec 20, 2023 at 3:42 PM Yang Shi <shy828301@xxxxxxxxx> wrote: > > > > On Tue, Dec 19, 2023 at 9:41 PM Fangrui Song <maskray@xxxxxxxxxx> wrote: > > > > > > Commit e6be37b2e7bd ("mm/huge_memory.c: add missing read-only THP > > > checking in transparent_hugepage_enabled()") introduced the VM_EXEC > > > requirement, which is not strictly needed. > > > > > > lld's default --rosegment option and GNU ld's -z separate-code option > > > (default on Linux/x86 since binutils 2.31) create a read-only PT_LOAD > > > segment without the PF_X flag, which should be eligible for THP. > > > > > > Certain architectures support medium and large code models, where > > > .lrodata may be placed in a separate read-only PT_LOAD segment, which > > > should be eligible for THP as well. > > > > Yeah, it doesn't have to be VM_EXEC. The original implementation was > > restricted to VM_EXEC to minimize the blast radius and the targe use > > case is for large text segments. Out of curiosity, did you see any > > noticeable improvement with this change? > > Hi Yang, > > Thanks for the comment. Frankly, I am not familiar with huge pages... > I noticed this VM_EXEC condition when I was writing this > hugepage-related section in > https://maskray.me/blog/2023-12-17-exploring-the-section-layout-in-linker-output#transparent-huge-pages-for-mapped-files > (Thanks to Alexander Monakov's comment about > CONFIG_READ_ONLY_THP_FOR_FS in > https://mazzo.li/posts/check-huge-page.html). Thanks for sharing the article, learnt something about linker and loader. > > As dTLB for read-only data is also an important optimization of > file-backed THP, it seems straightforward that we should drop the > VM_EXEC condition :) Yeah, as long as the use case is valid, it is definitely fine to lift the restriction. > > On my Arch linux machine, the r--p page gets split if I invoke > madvise(__ehdr_start, HPAGE_SIZE, MADV_HUGEPAGE) I haven't figured out > why it behaves so in the presence of the VM_EXEC check. What do you mean about "split"? THP got split into small pages? It depends on the address of __ehdr_start. If it is in the middle of a VMA, the VMA is going to be split due to the different huge page attributes. > > % g++ test.cc -o ~/tmp/test -O2 -fuse-ld=lld > -Wl,-z,max-page-size=2097152 && sudo ~/tmp/test > __ehdr_start: 0x55f3b1c00000 > 55f3b1c00000-55f3b1e00000 r--p 00000000 103:03 555277119 > /home/ray/tmp/test > 55f3b1e00000-55f3b1e01000 r--p 00200000 103:03 555277119 > /home/ray/tmp/test > 55f3b2000000-55f3b2002000 r-xp 00200000 103:03 555277119 > /home/ray/tmp/test > 55f3b2201000-55f3b2202000 r--p 00201000 103:03 555277119 > /home/ray/tmp/test > 55f3b2401000-55f3b2402000 rw-p 00201000 103:03 555277119 > /home/ray/tmp/test > 55f3b3a9a000-55f3b3abb000 rw-p 00000000 00:00 0 [heap] > > > It'd be greatly appreciated if someone familiar with > CONFIG_READ_ONLY_THP_FOR_FS could provide some notes on how to use > this feature:) I think your blog covered all the points. If you don't mind, you could add some notes in Documentation/admin-guide/mm/transhuge.rst. > > > > > > > Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx> > > > --- > > > include/linux/huge_mm.h | 1 - > > > 1 file changed, 1 deletion(-) > > > > > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h > > > index fa0350b0812a..4c9e67e9000f 100644 > > > --- a/include/linux/huge_mm.h > > > +++ b/include/linux/huge_mm.h > > > @@ -126,7 +126,6 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma) > > > inode = vma->vm_file->f_inode; > > > > > > return (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)) && > > > - (vma->vm_flags & VM_EXEC) && > > > !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode); > > > } > > > > > > -- > > > 2.43.0.472.g3155946c3a-goog > > > > > > > -- > 宋方睿