[PATCH] stash: reuse cached index entries in --patch temporary index

[PATCH] stash: reuse cached index entries in --patch temporary index

[Adam Johnson via GitGitGadget]

From: Adam Johnson <me@adamj.eu>

`git stash -p` prepares the interactive selection by creating a
temporary index at HEAD, switching `GIT_INDEX_FILE` to it, and then
running the `add -p` machinery.

That temporary index was created by running `git read-tree HEAD`.  The
resulting index had no useful cached stat data or fsmonitor-valid bits
from the real index.  When `run_add_p()` refreshed that temporary index
before showing the first prompt, it could end up lstat(2)-ing every
tracked file, even in a repository where `git diff` and `git restore -p`
can use fsmonitor to avoid that work.

Create the temporary index in-process instead.  Use `unpack_trees()` to
reset the real index contents to HEAD while writing the result to the
temporary index path.  For paths whose index entries already match HEAD,
`oneway_merge()` reuses the existing cache entries, preserving their
cached stat data and `CE_FSMONITOR_VALID` state.

This makes the refresh performed by `run_add_p()` behave like the one
used by `git restore -p`: unchanged paths can be skipped via fsmonitor
instead of being scanned again.

In a 206k file repository with `core.fsmonitor` enabled and a one-line
change in one file, time to first prompt dropped from 34.774 seconds to
0.659 seconds.

Signed-off-by: Adam Johnson <me@adamj.eu>
---
    stash: reuse cached index entries in --patch temporary index

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2306%2Fadamchainz%2Faj%2Foptimize-stash-patch-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2306/adamchainz/aj/optimize-stash-patch-v1
Pull-Request: https://github.com/git/git/pull/2306

 builtin/stash.c        | 71 ++++++++++++++++++++++++++++++++++++++----
 t/t3904-stash-patch.sh | 18 +++++++++++
 2 files changed, 83 insertions(+), 6 deletions(-)

diff --git a/builtin/stash.c b/builtin/stash.c
index 32dbc97b47..48189cb9f7 100644
--- a/builtin/stash.c
+++ b/builtin/stash.c
@@ -372,6 +372,57 @@ static int reset_tree(struct object_id *i_tree, int update, int reset)
 	return 0;
 }
 
+static int create_index_from_tree(const struct object_id *tree_id,
+				  const char *index_path)
+{
+	int nr_trees = 1;
+	int ret = 0;
+	struct unpack_trees_options opts;
+	struct tree_desc t[MAX_UNPACK_TREES];
+	struct tree *tree;
+	struct index_state dst_istate = INDEX_STATE_INIT(the_repository);
+	struct lock_file lock_file = LOCK_INIT;
+
+	repo_read_index_preload(the_repository, NULL, 0);
+	if (refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL))
+		return -1;
+
+	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR);
+
+	memset(&opts, 0, sizeof(opts));
+
+	tree = repo_parse_tree_indirect(the_repository, tree_id);
+	if (!tree || repo_parse_tree(the_repository, tree)) {
+		ret = -1;
+		goto done;
+	}
+
+	init_tree_desc(t, &tree->object.oid, tree->buffer, tree->size);
+
+	opts.head_idx = 1;
+	opts.src_index = the_repository->index;
+	opts.dst_index = &dst_istate;
+	opts.merge = 1;
+	opts.reset = UNPACK_RESET_PROTECT_UNTRACKED;
+	opts.fn = oneway_merge;
+
+	if (unpack_trees(nr_trees, t, &opts)) {
+		ret = -1;
+		goto done;
+	}
+
+	if (write_locked_index(&dst_istate, &lock_file, COMMIT_LOCK)) {
+		ret = error(_("unable to write new index file"));
+		goto done;
+	}
+
+done:
+	release_index(&dst_istate);
+	if (ret)
+		rollback_lock_file(&lock_file);
+	return ret;
+}
+
 static int diff_tree_binary(struct strbuf *out, struct object_id *w_commit)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
@@ -1321,18 +1372,26 @@ static int stash_patch(struct stash_info *info, const struct pathspec *ps,
 		       struct interactive_options *interactive_opts)
 {
 	int ret = 0;
-	struct child_process cp_read_tree = CHILD_PROCESS_INIT;
 	struct child_process cp_diff_tree = CHILD_PROCESS_INIT;
+	struct commit *head_commit;
+	const struct object_id *head_tree;
 	struct index_state istate = INDEX_STATE_INIT(the_repository);
 	char *old_index_env = NULL, *old_repo_index_file;
 
 	remove_path(stash_index_path.buf);
 
-	cp_read_tree.git_cmd = 1;
-	strvec_pushl(&cp_read_tree.args, "read-tree", "HEAD", NULL);
-	strvec_pushf(&cp_read_tree.env, "GIT_INDEX_FILE=%s",
-		     stash_index_path.buf);
-	if (run_command(&cp_read_tree)) {
+	head_commit = lookup_commit(the_repository, &info->b_commit);
+	if (!head_commit || repo_parse_commit(the_repository, head_commit)) {
+		ret = -1;
+		goto done;
+	}
+	head_tree = get_commit_tree_oid(head_commit);
+	if (!head_tree) {
+		ret = -1;
+		goto done;
+	}
+
+	if (create_index_from_tree(head_tree, stash_index_path.buf)) {
 		ret = -1;
 		goto done;
 	}
diff --git a/t/t3904-stash-patch.sh b/t/t3904-stash-patch.sh
index 90a4ff2c10..4b3241c8cd 100755
--- a/t/t3904-stash-patch.sh
+++ b/t/t3904-stash-patch.sh
@@ -84,6 +84,24 @@ test_expect_success 'none of this moved HEAD' '
 	verify_saved_head
 '
 
+test_expect_success 'stash -p with unmodified tracked files present' '
+	git reset --hard &&
+	echo line1 >alpha &&
+	echo line1 >beta &&
+	git add alpha beta &&
+	git commit -m "add alpha and beta" &&
+	echo line2 >>alpha &&
+	echo y | git stash -p &&
+	echo line1 >expect &&
+	test_cmp expect alpha &&
+	test_cmp expect beta &&
+	git stash pop &&
+	printf "line1\nline2\n" >expect &&
+	test_cmp expect alpha &&
+	echo line1 >expect &&
+	test_cmp expect beta
+'
+
 test_expect_success 'stash -p with split hunk' '
 	git reset --hard &&
 	cat >test <<-\EOF &&

base-commit: 7bcaabddcf68bd0702697da5904c3b68c52f94cf
-- 
gitgitgadget

Re: [PATCH] stash: reuse cached index entries in --patch temporary index

[Junio C Hamano]

"Adam Johnson via GitGitGadget" <gitgitgadget@gmail.com> writes:

> From: Adam Johnson <me@adamj.eu>
>
> `git stash -p` prepares the interactive selection by creating a
> temporary index at HEAD, switching `GIT_INDEX_FILE` to it, and then
> running the `add -p` machinery.
>
> That temporary index was created by running `git read-tree HEAD`.  The
> resulting index had no useful cached stat data or fsmonitor-valid bits
> from the real index.  When `run_add_p()` refreshed that temporary index
> before showing the first prompt, it could end up lstat(2)-ing every
> tracked file, even in a repository where `git diff` and `git restore -p`
> can use fsmonitor to avoid that work.
>
> Create the temporary index in-process instead.  Use `unpack_trees()` to
> reset the real index contents to HEAD while writing the result to the
> temporary index path.  For paths whose index entries already match HEAD,
> `oneway_merge()` reuses the existing cache entries, preserving their
> cached stat data and `CE_FSMONITOR_VALID` state.

Clever.  As the fsmonitor_valid bit is in-core only, updating the
index in-process would be an obvious and probably the only sensible
way to preserve it.

I however have to wonder if simply replacing the external process
invocation with "git read-tree -m HEAD" (i.e., oneway merge) gives
a similar speed-up.

> This makes the refresh performed by `run_add_p()` behave like the one
> used by `git restore -p`: unchanged paths can be skipped via fsmonitor
> instead of being scanned again.
>
> In a 206k file repository with `core.fsmonitor` enabled and a one-line
> change in one file, time to first prompt dropped from 34.774 seconds to
> 0.659 seconds.

Interesting.

> diff --git a/t/t3904-stash-patch.sh b/t/t3904-stash-patch.sh
> index 90a4ff2c10..4b3241c8cd 100755
> --- a/t/t3904-stash-patch.sh
> +++ b/t/t3904-stash-patch.sh
> @@ -84,6 +84,24 @@ test_expect_success 'none of this moved HEAD' '
>  	verify_saved_head
>  '
>  
> +test_expect_success 'stash -p with unmodified tracked files present' '
> +	git reset --hard &&
> +	echo line1 >alpha &&
> +	echo line1 >beta &&
> +	git add alpha beta &&
> +	git commit -m "add alpha and beta" &&
> +	echo line2 >>alpha &&
> +	echo y | git stash -p &&
> +	echo line1 >expect &&
> +	test_cmp expect alpha &&
> +	test_cmp expect beta &&
> +	git stash pop &&
> +	printf "line1\nline2\n" >expect &&
> +	test_cmp expect alpha &&
> +	echo line1 >expect &&
> +	test_cmp expect beta
> +'

What I read from the proposed log message is that the change is
purely about performance and should not change any behaviour.  Why
do we need a new test in t/t3904?  I would not have surprised if we
saw a new test in t/perf/, though.

Thanks.

Re: [PATCH] stash: reuse cached index entries in --patch temporary index

[Junio C Hamano]

"Adam Johnson via GitGitGadget" <gitgitgadget@gmail.com> writes:

>  2 files changed, 83 insertions(+), 6 deletions(-)
>
> diff --git a/builtin/stash.c b/builtin/stash.c
> index 32dbc97b47..48189cb9f7 100644
> --- a/builtin/stash.c
> +++ b/builtin/stash.c
> @@ -372,6 +372,57 @@ static int reset_tree(struct object_id *i_tree, int update, int reset)
>  	return 0;
>  }
>  
> +static int create_index_from_tree(const struct object_id *tree_id,
> +				  const char *index_path)
> +{
> +	int nr_trees = 1;
> +	int ret = 0;
> +	struct unpack_trees_options opts;
> +	struct tree_desc t[MAX_UNPACK_TREES];
> +	struct tree *tree;
> +	struct index_state dst_istate = INDEX_STATE_INIT(the_repository);
> +	struct lock_file lock_file = LOCK_INIT;
> +
> +	repo_read_index_preload(the_repository, NULL, 0);
> +	if (refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL))
> +		return -1;

Is this "non-zero return from refresh_index() leads to a failure"
intended?  The old "git read-tree HEAD" wouldn't have cared if the
original index were unmerged, for example, but with this update, we
will see an immediate failure.  There are other conditions that
refresh_index() flips its local variable has_errors on, which leads
to its non-zero return.

Since "git stash -p" is almost always invoked when the user has
unstaged modifications, I am not sure allowing refresh_index() to
notice and barf is what we want here.

Re: [PATCH] stash: reuse cached index entries in --patch temporary index

[Adam Johnson]

> I however have to wonder if simply replacing the external process
> invocation with "git read-tree -m HEAD" (i.e., oneway merge) gives
> a similar speed-up.

Good idea, I just tried this, but it does not help. The subprocess runs
with GIT_INDEX_FILE set to a temporary index, so oneway_merge
never uses the CE_FSMONITOR_VALID fast path.

The in-process approach is necessary because it lets us set
opts.src_index to the real index with cached stat and fsmonitor data,
before switching GIT_INDEX_FILE.

> What I read from the proposed log message is that the change is
> purely about performance and should not change any behaviour.  Why
> do we need a new test in t/t3904?  I would not have surprised if we
> saw a new test in t/perf/, though.

Ah yeah, my bad. I added this while iterating to catch a bug I introduced,
but it's not necessary for the final patch. Will remove.

On Wed, 20 May 2026, at 03:08, Junio C Hamano wrote:
> "Adam Johnson via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
> > From: Adam Johnson <me@adamj.eu>
> >
> > `git stash -p` prepares the interactive selection by creating a
> > temporary index at HEAD, switching `GIT_INDEX_FILE` to it, and then
> > running the `add -p` machinery.
> >
> > That temporary index was created by running `git read-tree HEAD`.  The
> > resulting index had no useful cached stat data or fsmonitor-valid bits
> > from the real index.  When `run_add_p()` refreshed that temporary index
> > before showing the first prompt, it could end up lstat(2)-ing every
> > tracked file, even in a repository where `git diff` and `git restore -p`
> > can use fsmonitor to avoid that work.
> >
> > Create the temporary index in-process instead.  Use `unpack_trees()` to
> > reset the real index contents to HEAD while writing the result to the
> > temporary index path.  For paths whose index entries already match HEAD,
> > `oneway_merge()` reuses the existing cache entries, preserving their
> > cached stat data and `CE_FSMONITOR_VALID` state.
> 
> Clever.  As the fsmonitor_valid bit is in-core only, updating the
> index in-process would be an obvious and probably the only sensible
> way to preserve it.
> 
> I however have to wonder if simply replacing the external process
> invocation with "git read-tree -m HEAD" (i.e., oneway merge) gives
> a similar speed-up.
> 
> > This makes the refresh performed by `run_add_p()` behave like the one
> > used by `git restore -p`: unchanged paths can be skipped via fsmonitor
> > instead of being scanned again.
> >
> > In a 206k file repository with `core.fsmonitor` enabled and a one-line
> > change in one file, time to first prompt dropped from 34.774 seconds to
> > 0.659 seconds.
> 
> Interesting.
> 
> > diff --git a/t/t3904-stash-patch.sh b/t/t3904-stash-patch.sh
> > index 90a4ff2c10..4b3241c8cd 100755
> > --- a/t/t3904-stash-patch.sh
> > +++ b/t/t3904-stash-patch.sh
> > @@ -84,6 +84,24 @@ test_expect_success 'none of this moved HEAD' '
> >  verify_saved_head
> >  '
> >  
> > +test_expect_success 'stash -p with unmodified tracked files present' '
> > + git reset --hard &&
> > + echo line1 >alpha &&
> > + echo line1 >beta &&
> > + git add alpha beta &&
> > + git commit -m "add alpha and beta" &&
> > + echo line2 >>alpha &&
> > + echo y | git stash -p &&
> > + echo line1 >expect &&
> > + test_cmp expect alpha &&
> > + test_cmp expect beta &&
> > + git stash pop &&
> > + printf "line1\nline2\n" >expect &&
> > + test_cmp expect alpha &&
> > + echo line1 >expect &&
> > + test_cmp expect beta
> > +'
> 
> What I read from the proposed log message is that the change is
> purely about performance and should not change any behaviour.  Why
> do we need a new test in t/t3904?  I would not have surprised if we
> saw a new test in t/perf/, though.
> 
> Thanks.
> 

Re: [PATCH] stash: reuse cached index entries in --patch temporary index

[Adam Johnson]

> Is this "non-zero return from refresh_index() leads to a failure"
> intended?

Good catch, it’s not needed. Removing, we can make the call
unconditional.

On Wed, 20 May 2026, at 03:26, Junio C Hamano wrote:
> "Adam Johnson via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
> >  2 files changed, 83 insertions(+), 6 deletions(-)
> >
> > diff --git a/builtin/stash.c b/builtin/stash.c
> > index 32dbc97b47..48189cb9f7 100644
> > --- a/builtin/stash.c
> > +++ b/builtin/stash.c
> > @@ -372,6 +372,57 @@ static int reset_tree(struct object_id *i_tree, int update, int reset)
> >  return 0;
> >  }
> >  
> > +static int create_index_from_tree(const struct object_id *tree_id,
> > +   const char *index_path)
> > +{
> > + int nr_trees = 1;
> > + int ret = 0;
> > + struct unpack_trees_options opts;
> > + struct tree_desc t[MAX_UNPACK_TREES];
> > + struct tree *tree;
> > + struct index_state dst_istate = INDEX_STATE_INIT(the_repository);
> > + struct lock_file lock_file = LOCK_INIT;
> > +
> > + repo_read_index_preload(the_repository, NULL, 0);
> > + if (refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL))
> > + return -1;
> 
> Is this "non-zero return from refresh_index() leads to a failure"
> intended?  The old "git read-tree HEAD" wouldn't have cared if the
> original index were unmerged, for example, but with this update, we
> will see an immediate failure.  There are other conditions that
> refresh_index() flips its local variable has_errors on, which leads
> to its non-zero return.
> 
> Since "git stash -p" is almost always invoked when the user has
> unstaged modifications, I am not sure allowing refresh_index() to
> notice and barf is what we want here.
> 

[PATCH v2] stash: reuse cached index entries in --patch temporary index

[Adam Johnson via GitGitGadget]

From: Adam Johnson <me@adamj.eu>

`git stash -p` prepares the interactive selection by creating a
temporary index at HEAD, switching `GIT_INDEX_FILE` to it, and then
running the `add -p` machinery.

That temporary index was created by running `git read-tree HEAD`.  The
resulting index had no useful cached stat data or fsmonitor-valid bits
from the real index.  When `run_add_p()` refreshed that temporary index
before showing the first prompt, it could end up lstat(2)-ing every
tracked file, even in a repository where `git diff` and `git restore -p`
can use fsmonitor to avoid that work.

Create the temporary index in-process instead.  Use `unpack_trees()` to
reset the real index contents to HEAD while writing the result to the
temporary index path.  For paths whose index entries already match HEAD,
`oneway_merge()` reuses the existing cache entries, preserving their
cached stat data and `CE_FSMONITOR_VALID` state.

This makes the refresh performed by `run_add_p()` behave like the one
used by `git restore -p`: unchanged paths can be skipped via fsmonitor
instead of being scanned again.

In a 206k file repository with `core.fsmonitor` enabled and a one-line
change in one file, time to first prompt dropped from 34.774 seconds to
0.659 seconds. The new perf test file demonstrates similar improvements,
with maen times for without- and with-fsmonitor cases dropping from 6.90
and 6.83 seconds to 0.55 and 0.28 seconds, respectively.

Signed-off-by: Adam Johnson <me@adamj.eu>
---
    stash: reuse cached index entries in --patch temporary index

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2306%2Fadamchainz%2Faj%2Foptimize-stash-patch-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2306/adamchainz/aj/optimize-stash-patch-v2
Pull-Request: https://github.com/git/git/pull/2306

Range-diff vs v1:

 1:  b228160cc4 ! 1:  8785572c4d stash: reuse cached index entries in --patch temporary index
     @@ Commit message
      
          In a 206k file repository with `core.fsmonitor` enabled and a one-line
          change in one file, time to first prompt dropped from 34.774 seconds to
     -    0.659 seconds.
     +    0.659 seconds. The new perf test file demonstrates similar improvements,
     +    with maen times for without- and with-fsmonitor cases dropping from 6.90
     +    and 6.83 seconds to 0.55 and 0.28 seconds, respectively.
      
          Signed-off-by: Adam Johnson <me@adamj.eu>
      
     @@ builtin/stash.c: static int reset_tree(struct object_id *i_tree, int update, int
      +	struct lock_file lock_file = LOCK_INIT;
      +
      +	repo_read_index_preload(the_repository, NULL, 0);
     -+	if (refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL))
     -+		return -1;
     ++	refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL);
      +
      +	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR);
      +
     @@ builtin/stash.c: static int stash_patch(struct stash_info *info, const struct pa
       		goto done;
       	}
      
     - ## t/t3904-stash-patch.sh ##
     -@@ t/t3904-stash-patch.sh: test_expect_success 'none of this moved HEAD' '
     - 	verify_saved_head
     - '
     - 
     -+test_expect_success 'stash -p with unmodified tracked files present' '
     -+	git reset --hard &&
     -+	echo line1 >alpha &&
     -+	echo line1 >beta &&
     -+	git add alpha beta &&
     -+	git commit -m "add alpha and beta" &&
     -+	echo line2 >>alpha &&
     -+	echo y | git stash -p &&
     -+	echo line1 >expect &&
     -+	test_cmp expect alpha &&
     -+	test_cmp expect beta &&
     -+	git stash pop &&
     -+	printf "line1\nline2\n" >expect &&
     -+	test_cmp expect alpha &&
     -+	echo line1 >expect &&
     -+	test_cmp expect beta
     + ## t/perf/p3904-stash-patch.sh (new) ##
     +@@
     ++#!/bin/sh
     ++
     ++test_description="Performance tests for git stash -p"
     ++
     ++. ./perf-lib.sh
     ++
     ++test_perf_fresh_repo
     ++
     ++test_expect_success "setup" '
     ++	mkdir files &&
     ++	test_seq 1 100000 | while read i; do
     ++		echo "content $i" >files/$i.txt || return 1
     ++	done &&
     ++	git add files/ &&
     ++	git commit -q -m "add tracked files" &&
     ++	echo modified >files/1.txt
      +'
      +
     - test_expect_success 'stash -p with split hunk' '
     - 	git reset --hard &&
     - 	cat >test <<-\EOF &&
     ++test_perf "stash -p, no fsmonitor" \
     ++	--setup 'echo modified >files/1.txt' '
     ++	printf "q\n" | git stash -p >/dev/null 2>&1 || true
     ++'
     ++
     ++if test_have_prereq FSMONITOR_DAEMON
     ++then
     ++	test_expect_success "enable builtin fsmonitor" '
     ++		git config core.fsmonitor true &&
     ++		git fsmonitor--daemon start &&
     ++		git update-index --fsmonitor &&
     ++		git status >/dev/null 2>&1
     ++	'
     ++
     ++	test_perf "stash -p, builtin fsmonitor" \
     ++		--setup 'echo modified >files/1.txt && git status >/dev/null 2>&1' '
     ++		printf "q\n" | git stash -p >/dev/null 2>&1 || true
     ++	'
     ++
     ++	test_expect_success "stop builtin fsmonitor" '
     ++		git fsmonitor--daemon stop
     ++	'
     ++fi
     ++
     ++test_done


 builtin/stash.c             | 70 +++++++++++++++++++++++++++++++++----
 t/perf/p3904-stash-patch.sh | 43 +++++++++++++++++++++++
 2 files changed, 107 insertions(+), 6 deletions(-)
 create mode 100755 t/perf/p3904-stash-patch.sh

diff --git a/builtin/stash.c b/builtin/stash.c
index 32dbc97b47..c4809f299a 100644
--- a/builtin/stash.c
+++ b/builtin/stash.c
@@ -372,6 +372,56 @@ static int reset_tree(struct object_id *i_tree, int update, int reset)
 	return 0;
 }
 
+static int create_index_from_tree(const struct object_id *tree_id,
+				  const char *index_path)
+{
+	int nr_trees = 1;
+	int ret = 0;
+	struct unpack_trees_options opts;
+	struct tree_desc t[MAX_UNPACK_TREES];
+	struct tree *tree;
+	struct index_state dst_istate = INDEX_STATE_INIT(the_repository);
+	struct lock_file lock_file = LOCK_INIT;
+
+	repo_read_index_preload(the_repository, NULL, 0);
+	refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL);
+
+	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR);
+
+	memset(&opts, 0, sizeof(opts));
+
+	tree = repo_parse_tree_indirect(the_repository, tree_id);
+	if (!tree || repo_parse_tree(the_repository, tree)) {
+		ret = -1;
+		goto done;
+	}
+
+	init_tree_desc(t, &tree->object.oid, tree->buffer, tree->size);
+
+	opts.head_idx = 1;
+	opts.src_index = the_repository->index;
+	opts.dst_index = &dst_istate;
+	opts.merge = 1;
+	opts.reset = UNPACK_RESET_PROTECT_UNTRACKED;
+	opts.fn = oneway_merge;
+
+	if (unpack_trees(nr_trees, t, &opts)) {
+		ret = -1;
+		goto done;
+	}
+
+	if (write_locked_index(&dst_istate, &lock_file, COMMIT_LOCK)) {
+		ret = error(_("unable to write new index file"));
+		goto done;
+	}
+
+done:
+	release_index(&dst_istate);
+	if (ret)
+		rollback_lock_file(&lock_file);
+	return ret;
+}
+
 static int diff_tree_binary(struct strbuf *out, struct object_id *w_commit)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
@@ -1321,18 +1371,26 @@ static int stash_patch(struct stash_info *info, const struct pathspec *ps,
 		       struct interactive_options *interactive_opts)
 {
 	int ret = 0;
-	struct child_process cp_read_tree = CHILD_PROCESS_INIT;
 	struct child_process cp_diff_tree = CHILD_PROCESS_INIT;
+	struct commit *head_commit;
+	const struct object_id *head_tree;
 	struct index_state istate = INDEX_STATE_INIT(the_repository);
 	char *old_index_env = NULL, *old_repo_index_file;
 
 	remove_path(stash_index_path.buf);
 
-	cp_read_tree.git_cmd = 1;
-	strvec_pushl(&cp_read_tree.args, "read-tree", "HEAD", NULL);
-	strvec_pushf(&cp_read_tree.env, "GIT_INDEX_FILE=%s",
-		     stash_index_path.buf);
-	if (run_command(&cp_read_tree)) {
+	head_commit = lookup_commit(the_repository, &info->b_commit);
+	if (!head_commit || repo_parse_commit(the_repository, head_commit)) {
+		ret = -1;
+		goto done;
+	}
+	head_tree = get_commit_tree_oid(head_commit);
+	if (!head_tree) {
+		ret = -1;
+		goto done;
+	}
+
+	if (create_index_from_tree(head_tree, stash_index_path.buf)) {
 		ret = -1;
 		goto done;
 	}
diff --git a/t/perf/p3904-stash-patch.sh b/t/perf/p3904-stash-patch.sh
new file mode 100755
index 0000000000..4cfce638be
--- /dev/null
+++ b/t/perf/p3904-stash-patch.sh
@@ -0,0 +1,43 @@
+#!/bin/sh
+
+test_description="Performance tests for git stash -p"
+
+. ./perf-lib.sh
+
+test_perf_fresh_repo
+
+test_expect_success "setup" '
+	mkdir files &&
+	test_seq 1 100000 | while read i; do
+		echo "content $i" >files/$i.txt || return 1
+	done &&
+	git add files/ &&
+	git commit -q -m "add tracked files" &&
+	echo modified >files/1.txt
+'
+
+test_perf "stash -p, no fsmonitor" \
+	--setup 'echo modified >files/1.txt' '
+	printf "q\n" | git stash -p >/dev/null 2>&1 || true
+'
+
+if test_have_prereq FSMONITOR_DAEMON
+then
+	test_expect_success "enable builtin fsmonitor" '
+		git config core.fsmonitor true &&
+		git fsmonitor--daemon start &&
+		git update-index --fsmonitor &&
+		git status >/dev/null 2>&1
+	'
+
+	test_perf "stash -p, builtin fsmonitor" \
+		--setup 'echo modified >files/1.txt && git status >/dev/null 2>&1' '
+		printf "q\n" | git stash -p >/dev/null 2>&1 || true
+	'
+
+	test_expect_success "stop builtin fsmonitor" '
+		git fsmonitor--daemon stop
+	'
+fi
+
+test_done

base-commit: 7bcaabddcf68bd0702697da5904c3b68c52f94cf
-- 
gitgitgadget